Left 4 Dead/Rooftop Finale

You start in the emergancy ward of the hospital, and need to reach the roof.

Paths
There is one path to the radio. However, this is the first time you need to bust open a grate and crawl through.

Traps

 * Once you summon a rescue team, you need to hold off for about "15 minutes". The attacking waves are guaranteed to have at least two tanks, they will come after the news chopper reports 10 minutes remaining and that you are his final pickup.