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. Both these routes bring you to a central chamber with a ladder down, ammunition and alternate weaponry. Proceed onward to reach the first trap, a maintence bridge.

The bridge is activated be a button on the second floor. You will need to defend against incoming enemies, until both bridge sections lower. One defence point is standing in the corner near the switch, which gives good coverage over the various attack routes.

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.

Take the exit, and walk outside to the next building, until you reach the safehouse.

Respawn points

 * At the lowering bridge. You can also find pain pills in this location as well.