Rooftop Finale

"Mercy Hospital, pick up the radio if you are there..."

- News Chopper 5 Pilot

"Our chopper's here, let's get the hell outta Dodge!"

- Bill

The Rooftop Finale is the fifth and final level of the No Mercy campaign. Upon reaching the hospital roof, the Survivors find a makeshift defensive position in one of the main structures and must use a radio to alert the helicopter to their presence. The pilot tells them to prepare themselves and to call again once they are ready. Once the second call is made, the helicopter will begin an approach to the helipad but the pilot warns that it will take some time get there. He quotes an estimated arrival time (which is periodically updated on the radio during the finale) that is a purely nominal value bearing no relationship to actual elapsed game time.

In the interim, the Survivors must hold out against hordes of Common Infected, several Special Infected and at least two Tanks. As soon as the second Tank dies, the helicopter makes its final approach to the partly destroyed helipad and the Survivors must sprint from wherever they are to board it and make their escape. The Survivors may trigger an additional wave of Infected by shooting at the rotors of the helicopter as it lands.

A third Tank and supporting horde will spawn immediately prior to the helicopter's arrival at the helipad which makes the Survivors' disengagement a potentially challenging task. In this regard, it has been reported that if this Tank is allowed to reach the helicopter before it departs, he can wedge his arm inside and prevent it from departing―at least until he is dispatched. There have even been instances where this Tank manages to get into the helicopter at the last second and knock all the Survivors out of the machine.

With all (or some) of the Survivors aboard, a cut scene rolls showing the helicopter flying off, going behind the hospital before coming around and passing above the camera. The camera then fades to black and the "credits" roll.

The Original Ending
In the Developer Commentary, it is revealed that there was meant to be an ending cutscene after the Survivors got on the helicopter. The pilot was meant to complain about how he was attacked while saving another group of people. Afterwards, he would have died from the Infection, causing the helicopter to crash near the toll road outside Riverside―thus setting the scene for Death Toll. Valve subsequently decided that this left players with an unsatisfactory ending to the campaign and cut it from the game but left its sound files in the sounds folder, though Crash Course eventually restored this outcome.

Or you can listen to them here.

Crash Course (later DLC campaign) is an adaptation of the above "infected pilot" scenario in which Zoey shot the pilot when he turns out to be an infected. Since the survivors cannot fly an aircraft, it simply crashed.

High Roof Method

 * When you climb up the ladder and first emerge on the roof, do not leave the building that you are on. Instead, select one person to go down.
 * Do not call the radio yet. Instead, the person who jumped down should grab a first-aid kit, and bring it back to the building. One at a time, each person stand on the very edge, and let the person heal them. Repeat this until the three left up top are healed.
 * The person who dropped down can now call the helicopter. The other three should stay on top, watching out for Smokers and Hunters, or other types of Special Infected, depending on what game type it is (L4D only has Boomers, Smokers, and Hunters), while L4D2 has Chargers, Jockeys, and Spitters that can cause more trouble than it used to.
 * The Tank can still get you from where you are. Be especially careful, because there isn't much stopping you from flying off the building and falling 30 stories to your death.
 * It's possible to return to the safe room from here. However, it means abandoning your teammate, essentially leaving them to die while you're safe somewhere down below. If you do decide to do this, be aware that the Tanks can still get to you, and they can break down the safe house's door. Find a good spot, and be ready to wait it out. Around the time the second Tank comes, though, you should start making a run for the helipad, as it's exceedingly difficult to run for it through hordes and at least one Tank which spawn when rescue has arrived.
 * There are also two hidden rooms that have breakable doors which you can go in the room and have good cover and you can just melee the Infected when they come but you have to be guarding the entrance to the room while meleeing them but be careful against the Tank.
 * On a related note, if you are planning on holding out somewhere on the lower roof area, and three players have dropped down but one got caught by a Smoker or pounced by a Hunter inside the elevator shaft, you can shoot through the elevator doors (underneath the ladder next to the helipad) and save them.

Radio Room Closet Method
There's a closet in the structure with the radio, near the windows and the cache of weapons and ammo. If everyone gets inside of the closet, their backs are covered, and covering the front is exceedingly simple.
 * Once you are on the rooftop, stocked up, and healed, get into the closet.
 * Two people should be near the door crouched and using melee, while the other two are behind them shooting, preferably with dual M1911s to conserve ammo.
 * Once the helicopter is called, all four Survivors should stay in the closet, until the first Tank appears. When the Tank appears, all four Survivors should leave the closet, and defeat the Tank like normal.
 * When the Tank is dead, return to the closet, and repeat with melee and shooting formation until the helicopter comes.
 * A good tip is to create a medium-sized hole through the closet door. This way, the players can shoot at the oncoming horde while having some extra cover.
 * The closet has been removed from the Versus and Left 4 Dead 2 versions of the game, but it is still in the single-player Campaign mode.

It is possible to do the Closet Method in single player mode. Although bots usually won't come into the closet, you can still hide out in the closet and provide covering fire for them. Then when the Tank comes get out of the closet and defeat it like normal.

Helipad Method
The ramp leading up to the helipad has empty space beneath it. The Survivors can easily fit under here, and take on the horde as it files in.
 * Once everyone is well-stocked, go under the ramp.
 * Crouch and back in as far as possible.
 * Have someone activate the radio, and then run back to you.
 * This area functions like the closet, except that you can shoot out through the sides and top (this functions as wall penetration, so damage will be reduced, but the Smoker can't hit you).
 * Once the Tank is spawned, it is advised that you leave the ramp until the Tank is dead. The Tank can easily hit all four Survivors if they are packed close together under the ramp.
 * As of January 14th, 2009, this has been patched on the PC version. The area under the ramp is now filled with crates. The Xbox 360 version has also now been given an update as of February 24th.

Stairwell Method
In the stairwell in the radio room, there are two doors. Station two people at each door: one person to crouch and melee, the other to shoot.
 * Make sure everyone is well stocked with ammo and such.
 * As long as everyone pays attention, Special Infected can't cause any problems.
 * When Tanks come, scatter, light it on fire, and play cat and mouse until it's dead.
 * When the helicopter comes, rush to the pad and get out of there.
 * It makes the finale easier if you place some gas cans, propane tanks, etc. around the pad so you can blow them as you leave, taking some of the Infected out.
 * Due to a level redesign of the Radio House on Versus and Survival, this stairwell is no longer sheltered by walls and is out in the open.

Glitch Ledge
On the highest area of the radio room, a Survivor can get on a small ledge by slowly walking off the roof of the building.
 * In using this area, none of the Infected can reach the Survivors, except the Hunter and the Smoker, but they can be easily dealt with.
 * If playing on one of the higher levels, one of the Tanks may have enough time to learn to climb onto your ledge, so make the best of your time!
 * The Untouchables achievement can be achieved through this method.
 * This ledge was removed in the Versus version, but a similar exploit exists on the high building to the right of the Minigun when facing the helipad. Be advised that human Tanks are more likely to toss rocks at you in this position. This has now been removed from the Xbox 360 version as of the February 24th update.

"Jesus Room" Method
Note: This method is considered an exploit. When you first climb onto the roof and face the helipad below, you'll see a building to the right of the helipad (the satellite roof). Go over to it, climb up the ladder and turn right. If you hop up onto this ledge while Infected are trying to get you from below and crawl to the very edge, the Infected won't be able to get up to you. If you position yourself directly above the door below, the Infected will break it open and you can enter the room beyond. The door can also be opened via constant melee, as the door in The Sewers level, but takes considerably longer. This seems to be a room that Infected spawn from during the finale, and if you hide at the very back of this room, no Infected, Special or otherwise, will be able to find you (unless they targeted you while you were entering or you're covered in Boomer bile). If you're patient, swarms will eventually die and the Tank will spawn. This is where it gets hairy. Sometimes the Tank dies on his own because he can't find you, but this doesn't seem to happen every time, so you may need to go out and kill him. Just remember that if you light him with a Molotov and retreat back into this room, he will be able to find you. Once the second Tank has died, you can run out to the helipad. With some luck, Untouchables can be attained with ease through this method.

"Blind Spot" Method
Note: This method is considered an exploit. There is a point in the map which is invisible to the AI―the only Special Infected that may get close enough is a Boomer or Smoker, although only the Smoker can attack until the players move. This will not work with bots, since the point appears to be outside their node graph as well.

When standing behind the Minigun, make a 90-degree left turn. Move forward until you reach the edge of the roof―you will pass between two L-shaped air-conditioning conduits. If you position yourselves between these with your backs to the wall, then you're outside the navigation layout for the map. All players must keep backing up against the wall since they will push each other out of place if just left.

You'll hear various hordes and specials spawn in, but none can reach you. If one of the players doesn't keep their back to the wall, then you might see a Smoker or a couple of Common Infected, but that's it. All finales consist of two Tank events―simply wait the first one out until he dies of frustration. Special Infected that spawn earlier in the battle will also be removed by the Director unless they are directly above/behind the Survivors.

For the second one, you have two options:


 * 1) Run to the helipad. Once the second Tank is dead, the helicopter arrives, so running to the helipad and killing the Tank en route means you'll have less Infected to fight. Contrary to normal tactics, you should run around the outside of the building here―the Tank will know where you are once you leave the blind spot, but as long as nobody does anything stupid, you shouldn't get hit.
 * 2) Wait it out. The Tank will eventually die from frustration, at which point the Helicopter will arrive. The problem is that as soon as the Helicopter arrives, so does the horde, Special Infected and so on. Run around the outside again, and throw pipe bombs once you get close to the helipad.

Note that in both these versions, if anyone didn't keep stuck to the wall, then a Boomer may be waiting above the blind spot on the ledge behind, ready to vomit as the Survivors leave it―you can hear him burping away, so the first player out should do a 180 degree and kill him so that the others don't get slimed.

Idle Glitch Method
Note: You will need at least two players to perform this method.

Perform the "Jesus Room" method above. Once in the "Jesus Room," go idle and your character will run into the corner, then just wait and you should be in the safe room you started in now. It is possible for your character to simply teleport somewhere else on the rooftop. If you do, just return to the "Jesus Room" and try again. You can wait near the elevator until the helicopter arrives and then you can make a run for it. This can be an easy way of getting the Untouchables achievement.

The Spotlight Area Method
Note: This method is considered an exploit. In the area of the spotlight that's at the Southeast edge of the roof, if you can land on the red area and if you go slowly back toward the South edge of the area, you will fall down slightly and it looks like you're in midair. Infected AI bots cannot get to you as long as your flashlight is off. Be careful sometimes the Smoker will be able to grab you. The Tank will not show up at all if its done correctly. After the second tank run for the Chopper. Note the survivor bots will just jump to the spotlight area and attract the infected to you if the bots are still alive the tank will spawn, so its best to kill them right after signaling the Chopper, best done on harder difficulties. It is possible with some luck to get the Untouchables achievement this way.

Note: In all instances when playing with bots there is a strong risk that individual computer-controlled characters will not 'stick to the script' and arbitrarily relocate themselves to an alternative position on the rooftop that will not only take them away from the protective gunfire of their colleagues but may also draw off a second bot to accompany them. In the event of a freelance bot being incapacitated by a Special Infected, the other bots will instantly go to the rescue thus causing a collapse in the defensive plan until (hopefully) the situation can be restored. For example, when mounting a Radio Room Defense, a favorite place for a bot to go to is the elevated structure to the left of the radio room. In this instance, a human player can make the best of the situation (since this bot is likely to attract large numbers of attackers to this position) by providing some degree of covering gunfire from the left-hand radio room window.

The "Minigun-Camping" Method (L4D2 only)
Note: This method is considered an exploit. If you jump on minigun and stand on it, you will find that this makes you almost completely invulnerable during the whole finale. The only threats are Smokers and Spitters may be able to spit on you (although she has to get quite close to do so). Tank usually won't spawn (you will only hear music for few seconds), but if the Director will spawn him somewhere close to radio building (i.e. under tower with Grenade Launcher lying on it), he may be able to find you and successfully attack. Best doing it with players only since bots will attract horde, make Tank able to find you and kill Special Infected, making a chance for AID to spawn Smoker which may ruin your tactic, but still you can try it. It is worth noting that you don't have to crouch on minigun, standing on it will make you safe too.

Left 4 Dead 2 Version
The map uses the Versus version from Left 4 Dead, even on co-op modes. As such, the staircase at the back is uncovered. Since there is no Rescue Closet, Survivors must be careful to not be killed at any point, especially before the finale, if they have a chance of escape (unless a player has a Defibrillator). Other than this, the only changes are that the supplies are now found on the roof of the radio room, and the addition of guard railings around the spot where survivors emerge on the roof, as this spot was deemed far too easy for an intelligent, human-controlled Charger to get an instant kill on the first survivor to climb up in Versus.

The Boomer
A Boomer is a valuable asset here, not only because it is a mixture of interior and exterior locations, but also because it is a finale. There are a few key things you need to keep in mind for this level:


 * After the Survivors have radioed the helicopter, you cannot choose where you spawn; instead, you will spawn in a random nearby location chosen by the Director. Because of this, it is often more effective to back off and find a place to sit and let your bile recharge, as you may lose an effective ambush position. If you're lucky and have good positioning, you can score multiple horde attacks by raining bile down upon them each time you recharge. This depletes their health with the horde, forces them to have to huddle together, and gives your teammates ample time to respawn and deal damage.
 * The Hunter can cooperate with a Boomer pounce on the Survivor covered in bile to add terrible confusion to the mess; the bile will summon more Infected, who will create a meat shield for the Hunter. By the time the other Survivors are able to get to their pinned friend, it may be too late.
 * The Smoker can constrict a Survivor who has been covered, effectively creating a piñata for the Common Infected. This is a very effective way to incapacitate a Survivor quickly, especially since the other Survivors are likely to be busy.
 * The Tank can distract those who are not covered in bile, cutting them off from helping their friend. It can also incapacitate a covered Survivor, rendering them nearly helpless against the Infected onslaught all over them and adding the chance that they'll bleed to death.
 * Boomers are large, making them easy targets. If you manage to vomit upon a group of Survivors in a place where running away is going to be difficult, and finding cover will take some time; don't be afraid of running in and attacking them with your melee attack. This will deal extra damage. While in the middle of the finale's chaos, it can also confuse the Survivors from which direction the horde are actually attacking them from. Also, if they're not careful, they can shoot you once they're clean, resulting in being covered again.
 * If you are going to try the method above, run to someone near the ledge. If you are lucky enough, not only that you will get them covered, but they may fall off the ledge.

The Hunter
Although you are outside, the Hunter is not completely at home. However, even here, the Hunter can cause some serious damage, especially during the chaos of the finale. There are a few key things to remember:


 * The Hunter's bonus pounce damage is based on the distance between your launch-point and the point where you hit the Survivor; therefore, launching straight up or down is not the most effective pounce. Instead, climb on top of an air-conditioning unit or a nearby high ledge a bit away from the Survivors, and propel yourself up and forward at a 45° angle. You have some limited control in the air and can somewhat guide yourself to your target once airborne. Twenty-plus-damage pounces are not as hard as they seem once you become better acquainted with the pounce mechanic, 25 extra damage is the maximum.
 * When the finale's horde has surrounded and is attacking the Survivors, it is a very good idea to simply claw away at them. The Survivors might melee you once or twice, but they will have an extremely hard time identifying you as a Special Infected, and you can easily shred away quite a bit of health. This also works well with Survivors covered in bile, as they cannot see you.
 * If the Boomer "slimes" three people, it is better to pounce the clean Survivor as being "slimed" decreases vision and the Infected meat shield will prevent them from getting to you. The Boomer's victims are unable to see the aura surrounding their teammates while they're covered, so they won't be able to see the pounced victim until the initial blindness wears off.
 * When the Smoker grabs someone, don't melee the Survivor who is being strangled by him, as your claw will break the tongue and free the victim. Also, do not kill steal from Smokers by pouncing on a grabbed Survivor. Protect them by pouncing would-be rescuers.
 * The Tank will most likely inadvertently kill you if you pounce his close and immediate target. Instead, try to go for someone who is either not running from the Tank, or someone who you don't think the Tank will attack first.
 * If you're lucky enough to land on a Survivor close to the edge at the right time, you might fall to the lower parts of the rooftop and kill them due to the height.
 * Another useful tactic as a Hunter is to pounce on a Survivor that is climbing the ladder to the rooftop, causing him/her to fall down the ladder (the fall can cause plenty of damage to the Survivor).

The Smoker
The Smoker is designed to separate the group and make it difficult for the Survivors to withstand horde attacks. Here, there are certain points you need to bear in mind:


 * The Hunter, despite having the ability to steal the Smoker's target, can work together with the Smoker in many ways:
 * Between them, they can immediately incapacitate two separated Survivors without a fight.
 * Confident Survivors will usually chase after a fleeing Smoker whose tongue is recharging in order to kill them off. If you notice this behavior, take advantage of it. Have a Hunter wait around a corner, then get one of the Survivors to chase you, and have the Hunter pounce the Survivor chasing you. If you're lucky, by the time help arrives for the Survivor, your tongue ability will have recharged.
 * Hunters can aid Smokers by lying in wait. Once the Smoker has snared a target, the Hunter can then pounce the Survivor trying to save the snared Survivor.


 * If a Boomer has vomited on the three other Survivors, you can take the opportunity to grab the fourth. While the other three are covered in bile, they cannot see aura signals of their allies. In the mess of the finale battle, the lack of an aura means they may not even notice. Plus, once they do notice, your target will most likely be far away and in bad shape.
 * The Smoker is the Tank's best friend. The last thing a group of Survivors needs is to have one of their numbers dragged a long distance off during a Tank battle. You have two Tank battles during the finale to help the Tank to defeat the Survivors. People say that the Smoker is a harmless zombie, but they are very wrong. A Smoker, in combination with a Tank, can spell death for the Survivors. All you need to do is get the Smoker to drag away the person with the Molotov. If there are multiple people with Molotovs, then get a Hunter to take your target because it's far away from the fight and then take someone else. If a Boomer gets your target, then you're in a lot of luck as you and the Hunter have killed two Survivors. But if there are only two Survivors left, then allow the Hunter to get your target as they do a bit more damage. The Smoker, in the right hands, can be a very formidable foe.

The Charger
The Charger's a powerhouse and can dish out some serious damage if used right. Especially in a finale like this since the Survivors on right on top of a rooftop with a lot of vulnerable spots. Here are some tips to maximize your damage:


 * When the Survivors are at the open elevator, a Charger can easily charge someone right off and kill them instantly and at the same time knock the others down. But remember to act fast because if you don't, then you can miss them fast.
 * When the Survivors are on top of the roof, a well-placed charge can take them down all the way down to the bottom part of the roof and, in some rare cases, right off the roof!
 * Once the Survivors have summoned the rescue and are camped in a spot, it can be rather difficult to pull off a really "good" charge. But that doesn't mean you can pull off a high-damaging charge. If the Survivors are camped on top of any of the small building, try to knock them off so the other infected can start to pick on the other Survivors.
 * Remember, Spitters are the Chargers' best friend. If there any human-controlled Spitters, have them wait until you charge one and have them spit at the victim so you can maximize the damage you two do.
 * Jockeys can help a Charger take a player. First have that Jockey make someone fall onto the ledge. As other players go to that Survivor, charge at them to knock others, and it might cause a fellow Survivor (depending on how close he is), to fall down with the one your'e charging at.
 * When the Survivors are running up the ramp to get to the chopper this is your last chance to strike. Charge them while they're on the ramp and you should be able to carry one off the ledge (this is also a great way to get the "Long Distance Carrier" achievement.)
 * The rightmost pipe bridge (when looking towards the chopper pad) from the radio building's upper level is a great spot to instantly kill a Survivor. It is risky if multiple survivors are present to spot you, as you need to get up onto the roof of the building where the supplies are, and Charge the survivor off the right-hand side of the building (again, when looking towards the chopper pad) as they are standing on this pipe bridge. Survivors should always be wary of Chargers before crossing this spot.

The Jockey

 * His lunge attack can force Survivors to backtrack.
 * Your lunge attack can cause Survivor to fall down, waiting for a bot/player to get them back up. This can help your fellow Infected target them more, as other Survivors will protect the savior from other Infected who will try to take him out.
 * The lunge does not make the Survivor fall to their death, unless the Survivor isn't resisting, so if you don't want to wait for thirty seconds, don't do it, unless you have a good human-controlled Hunter, Charger and Smoker.
 * Take advantage of your control over the survivors — you may have the chance to ride a survivor over a ledge for an incap or kill.

The Tank
The Tank has three opportunities to crush the Survivors during the finale; however, the last is simply stopping the Survivors from escaping. In order to do so, you will need to remember a number of things:


 * Your rock throw is usually very underestimated. With practice, it can be devastating and deal a lot of damage to the group of Survivors quickly. As a rule of thumb, aim your reticule above, and perhaps ahead of, the target while throwing.
 * If you incapacitate a Survivor, immediately re-target another. If someone has managed to get around you to help their incapacitated friend, show the helpful Survivor their mistake by knocking them out as well.
 * You do have the ability to kill instantly by hitting the Survivors off the side of the building and making them fall 30 stories to their death.
 * Do not attempt to help a Hunter or a Smoker with their trapped target. You will end up killing the Hunter or freeing the Smoker's victim respectively. You can, however, use them as bait for other Survivors and attack those who try to rescue their fellow Survivors from them.
 * A good method is when just as the helicopter opens the door, jump in the helicopter. They don't have room to move much, so you can slam them all in the helicopter.
 * Avoid being set on fire as best you can.
 * Take notice of who has a Molotov before you become the Tank, and avoid these players until the fire they throw burns out.
 * If you are dead when a Tank spawns, make sure he is aware of where the gas cans are located, as well as the Molotov carriers.
 * If you hit a Survivor you see has a Molotov, be sure to incapacitate them if you can. Otherwise, tell a teammate to incapacitate that player and chase the others.
 * If you see somebody about to throw fire, change direction immediately! If worst comes to worst, throw a rock to hit Survivors.
 * Fire kills a Tank after 30 seconds, and it shows the damage in your health.
 * If you are set on fire, charge the Survivors. You won't have much time to do your damage, so don't hold back.
 * When the helicopter arrives, though, don't worry about the fire. They will get to the helicopter fast, so don't hold back if your enemies made a blockade of fire; just charge through.
 * Never punch the last Survivor when in front of the helicopter, for this may cause the helicopter and the Survivors (including the one you punched) to escape causing the game to end.

Survival
Radio Rooftop Method


 * The roof with the minigun serves as a fairly effective location to defend against the Infected onslaught. Not only do you have the Minigun at your disposal, but there are pills on this roof as well. Facing away from the Minigun, the left of the air-conditioning unit in front of you is a good place for the Survivors to bunch up and buckle down. From here, Infected generally only come at you from the front, although some will clamber over the air-conditioning unit to attack you. Hunters need to get close to pounce, and only occasionally will a Smoker succeed in landing a well-aimed tongue snare. The Tank is the biggest problem with this method. However, if he comes from the front, he can be heavily wounded by use of the Minigun. If he doesn't, you're in for a tougher fight as you will have to scatter, defeat the Tank and return to the position all before the next wave shows up.

Building Outskirts Method


 * By moving down on to the lower levels of the roof, the Survivors can funnel the Infected to come from only two directions. Two Survivors should face one way, and two should face the other. Strategic use of gas cans can help with this method. Boomers generally cannot get close enough before getting a shot to vomit on the Survivors and Smokers are easily exposed and dealt with by your teammates. However, it is crucial that you do not stay near the actual edge itself. This is to prevent not only accidentally stepping off, but also to prevent the Hunter's pounce from stumbling you over the edge. When the Tank appears, have each two Survivors check their sides. When one discovers that it is coming on their side, they should start shooting at it until it gets close. Then they should fall back and regroup with the other two. As the Tank arrives there, the four Survivors should shoot at the Tank. If necessary, the Survivors may need to fall back further. However, fully concentrated fire should kill the Tank easily. Once the Tank is slain, return to your positions and continue to fight. Repeat this until you become overwhelmed.

Scavenge
This map supports Left 4 Dead 2's Scavenge Mode, with 21 gas cans, one of the highest numbers of gas cans on a map. This is difficult to complete due to the danger of falling or being hit off the building.

Behind the Scenes
In the earliest pictures of the Rooftop Finale, it seems that there was a building in front of the supply room that had electric generators in it. The supply room itself had a much different shape, with doors on its side. There was also a tower on top of the supply room. The building the player emerges from when he/she enters her rooftop area was much thinner.

Later versions of the map show removed the generators and tower with the familiar supply room. However, the ladder that leads to the second floor of the supply room has boxes under it, forcing the player to jump on the boxes before using the ladder. In addition, the building to the right of the supply room does not have stairs that connect it to the main floor. The interior of the supply room looked run down compared to the final version.

Design-wise, it seems that the sky was very dark in NM5 at the start, possibly to show that time has passed since the start and end of the campaign. The buildings in the background were all sprites in a 3D skybox, whereas in the final, the buildings are mostly brushes and models.

An early screenshot of it found in the game's files shows that, at one time, you were supposed to be able to see the streets surrounding Mercy Hospital. This was likely removed for optimization reasons. It also shows that there were a lot of buildings with lights on, which fits with other screenshots of No Mercy. In some of the pictures, it seems that there was a stadium in the background, which means that there was most likely a sports event going on at the time of the outbreak.