So, I need a horde to come and attack when a door is open’d. How?
A L4D zombie horde?
Are you trying to make it a panic event?
Yeah, yeah, I am.
I don’t have l4d, but assuming alienswarm is kind of the same.
I believe Firegod got it spot-on.
You can always go here for more detail: http://developer.valvesoftware.com/wiki/L4D_Level_Design/Panic_Events