This is a new version of 20 Min 'Til Evac, dubbed v1.2. A few improvements were made:
1. Overall cleanliness of certain areas of the map were fixed. Big fix in the main inside, and a few fixes on the bridge.
2. Anti-cheat system for the bridge was improved to make sure ANYONE that is on the bridge will die from the killballs after two minutes.
3. Zombie teleporters have been reworked in a few ways, cleaned up zombie spawn area.
4. Added a new gametype for use on the map, based upon Jakob Hunter's Pandemic gametypes. It is entitled "20 Min Experimnt." More details below.

As of now for v1.2
1. Fixing up the bridge aesthetically.
2. Fix gametype problems
3. Fix a problem with the number one teleporter not opening.

Welcome now to a (new) Infection Survival map. This game is based off of a game that I helped make popular on Halo 2 that was played on Headlong. The base that all the humans spawn at is modeled after the building used in the game on Headlong. It has now been cheat-proofed compared to my several old versions, on several different maps, v1.1 has improved the cheat-proofing.Now begins the story....

A UNSC Team of soldiers has been attacked by zombie creatures. They were driving down a tunnel, when they were attacked, and many were slaughtered. The vehicles have been made useless, and unusable. We now have to stay in an ancient building, and wait for Evac. They say it will arrive in 20 minutes, but I don't think we can last that long. We must try, but the zombies pop out of nowhere, and they seem to come from everywhere.
This is Sergeant Conner
If you're listening to this, you are the UNSC

Okay, not really, just wanted to throw in some Terminator jokes ^_^
So if you didn't understand that, here's a basic idea of the game:
Zombies spawn in the crypt. They wait 45 seconds before the door is opened and they are allowed to choose from 4 teleporters. Each teleporter takes them to a different place around the human-held base. One leads to a small building that has a teleporter to the Third Floor of the human-held base, but can only be used in v.1.1 after two minutes of gameplay have occured.

Humans spawn on a broken bridge. Note that I wouldn't move when I spawn, as a giant block shortly blows up a warthog. If you die, it doesn't matter, you still respawn in the building. Humans have to leave the bridge within two minutes, as killballs envelop the bridge, killing any cheating campers. Yes, the bridge is CHEATING, once the game starts. Why do you spawn and are already cheating? Because it is awesome.

The humans spawn with Magnums and Energy Swords, the Alpha Zombie spawns with a Gravity Hammer and an Energy Sword, and Zombies spawn with poor camo and Energy Swords. Humans can pick up the scattered Rifles around, the map hosts a few battle rifles and carbines, but be careful with the ammo, there isn't much of it.

So the humans have 20 minutes to survive, and then they get evaced...actually the game ends, but technically you win. If you DO legitely survive 20 minutes, I want to see that video.

The building is constructed with 3 levels, one with a hallway on the side, one with deployable covers to help block doors and a zombie lift that leads up to the level, and the top floor, which features a hole that can be used to see the rest of the floors, and holds to a balcony with a teleporter on it. The teleporter can be accessed by zombies from using the teleporter on the small building, which is accessible again after two minutes of gameplay from the zombie spawn.

Okay, so we have a new gametype, called 20 Min Experimnt. This is based upon Jakob Hunter's Pandemic gametypes. Basically, both zombies and humans are harder to kill, and because of the difficulty of the original gametype, makes the game easier to play. It may still be buggy, as it wasn't originally made for this map.
Now for downloads.
DOWNLOADS
20 Min 'Til Evac (Map)
20 Min 'Til Evac (Map without effects)
20 Min 'Til Evac (Gametype)
20 Min 'Til Evac Experimental (Experimental Pandemic gametype)

Strategies I've seen used:
1) The top floor has the least amount of doors. There are only 3 ways to get in, two being easy to see and block, and one that comes up below. There is a hole on the top floor for seeing onto the Second and First Floor, which is helpful for watching for zombies, but can also be used by zombies to jump up and get you. This is possibly one of the best spots to hide, yet zombies can use the Gravity Hammer through the floor, so be careful.
2) The hallway on the First floor is a good spot, but presents a small amount of space. There are only two doors, but it may be hard to see if zombies come in, as they can pop in quickly.
3) The Balcony can be used well for hiding. Zombies can hit you if you're near the edge, so you need to be careful. There are 2 doors, and one teleporter to this area. You can block the teleporter easily with your body, and make it hard for zombies to enter, and easy to pick them off when they appear outside the zombie building.

And so the epic pictures begin...

Overview of the map

Spawn Bridge for the Humans. Remember to leave quickly, but watch the very beginning spawn, the warthog blows up.

The bridge enveloped in anti-cheating goodness

First Floor

Zombie Hallway

Two Zombie Spots

Second Story

Third Floor

Third Floor 2

Zombie Spawn area.

I will name the teleporters for you:
1. Top left: Building teleporter (note that it only opens up after two minutes of play)
2. Bottom Left: Zombie lift to second Floor
3. Top Right: Front of the building
4. Bottom Right: Bottom level, hallway
Now when you refer to the teleporters, they are now 1-4, rather than "ummm the one that leads to that one place?"

The only cheating I can notice is that you can continually commit suicide, but most of the spawns are inside, so it's easy to catch cheaters. Also, the bridge is a pretty good cheat spot, but that only lasts for two minutes before killballs spawn and kill you. It shouldn't be possible to get on the roof, maybe only as a zombie.

Please post any (non-stupid) comments and concerns below. I'd like to make plenty more maps using this gametype, but I want this one perfect first. Also, I'd like to again give credit to Jakob Hunter for his gametype, it was so amazing I edited for 20 Min 'Til Evac.