No Mercy

No Mercy is the first campaign in Left 4 Dead and spans five levels.

The first level picks up where the game's introduction cutscene leaves off. The Survivors hear from a passing rescue helicopter they can be evacuated if they make their way to Mercy Hospital. On their trek to safety they encounter Infected-filled apartment buildings, city streets, subway tunnels, and sewers. They discover that Mercy Hospital has been overrun but that there is still a chance of rescue if they can fight their way to the helicopter landing pad on the roof of the hospital.

Below is a complete list of video walkthroughs of the 5 levels:

and 2
 * 1) The Apartments
 * 2) The Subway
 * 3) The Sewer Parts 1
 * 1) The Hospital
 * 2) Rooftop Finale

Please know that the gameplay videos are on each campaign's info page not on each map page.

Official description
The No Mercy campaign finds you on the rooftop of an obliterated apartment complex, deep in the heart of the once-thriving metropolis of Fairfield—now completely overrun with hordes of Infected. A voice from a bullhorn calls out from a rescue helicopter: Get to the roof of nearby Mercy Hospital and be airlifted to safety. Even if it means shooting your way through narrow alleys, open sewers and seething waves of zombies to get there.

Left 4 Dead 2
With The Sacrifice and Cold Stream DLCs, No Mercy was edited and ported over to Left 4 Dead 2. Thus it included all the new Infected, items and weapons, giving the campaign a whole new experience. Video walkthroughs of the Left 4 Dead 2 version of the campaign and all the changes:
 * 1) Maps 1 and 2
 * 2) Map 3
 * 3) Maps 4 and 5

Graffiti
The Apartments level contains plentiful graffiti. Much of it is located in the safe room and on walls along the way.

The captions are of a despairing nature, for instance, "Hope" is spray painted in large letters to the left of the doorway but someone has added "NO" above it to change it to say "No Hope". "Morbid", "None of you will survive", and "Entire city is Infected, if you can read this, get out" add to the aura of hopelessness.

In the finale, a clue as to Green Flu's nature is provided a written conversation on the wall recording the rapidly condensing passage of time between being bitten and turning into an Infected falling from 5 days to 3 days, 1 day, 2 1/2 hours, and then finally 5 minutes.

Demo
The Left 4 Dead demo consisted of The Apartments in its entirety, as well as most of The Subway. The rest of the Subway map could be seen in the demo if players went into Noclip mode.

Behind the Scenes
No Mercy's lighting effects underwent marked changes during development. Early on, the contrast between light and shade was accentuated. Ambient light in each scene was suppressed making for a dark and ill-lit environment and where there was a light, it was made extremely bright.

Skyscrapers and other buildings in the background appeared unnaturally flat because they were a part of the skybox texture. Since this does not affect gameplay, this may have been done deliberately to speed up the game's development for playtesting purposes. In the final version buildings are rendered as actual brushes and models.

The sky was once a dark blue as compared to the blue/green mix in the final version.

Buildings had most of their lights on. In the final version, only scattered windows are illuminated.