20 Min 'til Evac is a game originally played on Headlong in Halo 2. It is an infection game that was played in one of the buildings on Headlong, and it was very fun compared to normal infection on any other maps.
What makes this fun is the multiple ways to enter into the building. The building itself has 4 entrances, and other ways may be found as well. A list of the normal entrances are below.
The game is played by the humans running to the building (If you are confused on which building, see the screenshot below) and trying to hide inside of it for a full 20 minutes without dieing. Surviving for that long is very challenging, and after a while, zombies will become aware of your strategies, compared to other infection games.
There are 3 floors to the building, but the second and third floors are the safest. Zombies may enter the playing field from the sky bubble after 45 seconds, when the blocks are removed so that they may enter. The zombies enter the playing field through 5 different teleporters in the sky bubble, and the 6th one that leads straight to the building after 3 minutes. After the zombies die once, they can spawn onto the playing field, or back in the sky bubble.
Zombies have no shields, allowing them to be killed very easily with a headshot, or a melee. Alpha zombies are fully visible, but zombies have poor camo. Zombies spawn with only swords, Alpha Zombies with swords and hammers. Humans spawn with magnums and swords, but can pick up carbines or battle rifles littered around the playing field and some inside the building.
The weapon layout is supposed to match that on Headlong, although some of the carbines were switched out for Battle Rifles.
There is only one honor rule: The humans must stay in the designated building during the game at all times, so that it is fair for the zombies. Hiding in the dunes or the edge of the playing field is cheating!
Entrances:
1) Front Door
2) Back Door
3) Elevator
4) Top Teleporter
Open the spoiler to show the entrances on the overview picture.

Download
20 min 'til Evac (map)
20 min 'til Evac (gametype)

Screenshots:

Overview (This is the main building that the humans should stay in.)

Bottom Floor

Bottom Floor 2

Hallway

Second Floor

Elevator room

Stairway

Third floor

Third Floor 2

Zombie Spawn (Watch the mancannon, if you jump you die. This is also a picture after blocks have been removed.)

Teleporters (A bridge to the far left one is spawned after 3 minutes.)

Teleporter building (This is a second building for zombie's use only)

Actions Shots:

Don't play with the deployable cover that much...

Headshot = 1 hit kill

Watch the window for zombies, they can slash through it

I will be adding in screenshots of what the original map looked like on Headlong later, so that you may get an idea of how it looks like it.