No Mercy

No Mercy was the first campaign in Left 4 Dead and spanned five levels.

The first level picked up where the game's introduction cutscene left off. The Survivors heard from a passing rescue helicopter that they could be evacuated if they could make their way to Mercy Hospital. On their trek to safety, they encountered the Infected-filled apartment buildings, the city streets, the subway tunnels and the sewers. They discovered that Mercy Hospital was overrun, but there was still a chance of rescue if they could fight their way to the helicopter landing pad on the roof of the hospital.

Below was 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 and not on each map page.

Official description
The No Mercy campaign found you on the rooftop of an obliterated apartment complex that was deep in the heart of the once-thriving metropolis of Fairfield that was completely overrun with the hordes of Infected. A voice from a bullhorn called out from a rescue helicopter: Get to the roof of nearby Mercy Hospital and be airlifted to safety even if it meant shooting your way through the narrow alleys, the open sewers and the 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. It included all of the new Infected, items and weapons, giving the campaign a whole new experience. These are the 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 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 finally 5 minutes.

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

Behind the Scenes
No Mercy's lighting effects underwent marked changes during the 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.

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

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

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