Left 4 Dead/The Sewers

Paths
From the exit, turn left to the next building to find the only openable door.

You can enter the warehouse either through the ground floor, or by jumping through a second floor window.

The sewers in this map have many side tunnels in addition to the main pathways. You can take these tunnels to avoid being swarmed or instead go through the main pathways for easier navigation.

Traps

 * After the store, you need to activate a noisy lift. At this point, the horde attacks fast and hard, forcing you to take shelter in the next building. (Note that activating the lift doesn't strand survivors on the first floor - the lift also extends a ladder.)