Left 4 Dead/The Drains

Paths
From the safehouse exit, you can either go through the large pipe to the right, or through the large pipe opposite of the exit.

After lowering the bridge, the exit pipe has two branches. Pick the second one only if there's a large horde idling in the first path (and you didn't alert said horde.) At the end of the pipe, you reach a building - there is a ladder to the second floor in the first room, but you may be more comfortable walking to the stairs and climbing those.

Traps

 * You need to close floodgates in order to make a bridge across the drain. A siren will alert humans to get out of the way of a lowering bridge, as well as alerting the horde to human activity.