Foundation

Foundation is a Multiplayer level in Halo 2, based on the Thunderdome levels from the Marathon series.

Summary

Foundation is a symmetrical map with two levels; a lower center area and a raised walkway around the edge. There are rooms jutting from the corners of the upper walkway. The proportioned all-around tiers converge on all sides of one another, making a "circle" from a Bird's Eye view.

Weapon Locations

The layout of weapons on this level is symmetrical, like the map itself. An Energy Sword sits directly amidst four pseudo-buildings in the center. On the lower level, two Rocket Launchers sit beneath platforms opposite each other. There are holes in these platforms to allow players to drop down to the rocket launchers, while an airlift to either side of the launcher allows access back to the top. On the other sides of the ground level, Shotguns sit between two staircases, with a Magnum available on the stair. On the small platform above the shotguns, a Needler under each magnum, a Battle Rifle and rocket launcher ammunition can be found. Within the "spur" rooms on the upper level, SMG's can be found.

Territories

Arena
Lab #1
Lab #2
Lab #3
Lab #4

Tactics

First off, this is a symmetrical level. This makes the level relatively easy to navigate and memorize, so make sure to put in the minimal effort required to know all the features of the map.
By default, there are turrets here. Turrets are not recommended, because unless you have someone watching your back, you're likely to get assassinated. Players can come up the lifts behind you and kill you before you have the chance to get off. A reminder: the rockets on the map can be used to lock-on to turrets so there is practically no chance for a rocket to not kill you if you are using the turret.
There are only two ways up to the higher level (aside from spawning)- the stairs and the airlifts. If you can get bearings on each of these, no one can get to the top level.
In objective gametypes you can throw the object on the outside of the map when you are at the peak of your jump while going up the airlift. If done right, the object will be on top of the edge of the map and will be unable to be picked up. Do this if you are winning and contest the object to get a sure win.
In one of the bases there is a group of boxes. Place them in front of the door, grab that rocket launcher near the vent and hide behind the boxes. Wait for someone to come wandering in and finish them off.
You can drive a vehicle such as a Spectre or Ghost up the airlift. These are usually ineffective on ground level due to turret fire, but on top this threat is nullified and you can use the narrow walkways to your advantage.
Close combat is common, therefore be prepared: always arm yourself with the Energy Sword or Shotgun.

Unlocking Foundation

Early in its release, an auto update was released that made Foundation available for all Xbox Live users. For non-Live users, however, Foundation remains hidden away thanks to a bug in the game. Bungie's intent was for Foundation to appear after you completed the game on any difficulty. Unfortunately, Foundation was only unlocked if you completed a "training event" on the final level (The Great Journey). This was fixed with the release of the Multiplayer Map Pack on July 5th. Another way to unlock Foundation is to:

Create a new account.
Start a co-op game with an account that has unlocked The Great Journey level.
The new player must play to the final part of the level, where you fly a Banshee, then perform a Banshee stunt (some say that you need to make a roll to the left, a roll to the right and an upper roll ^{[citation needed]}).
Complete the level.

Foundation should now be unlocked whenever you play with the account you created.

Trivia

Foundation is the only Multiplayer Level in the Halo Trilogy that must be "unlocked".
There is a symbol of a person riding a bomb on a door on ground-level. There is also a picture of a woman on a ripped piece of paper on a door on the ground level. The woman is completely "darkened" out. Some say this might be another Easter egg.
When "Tactical Autonomous Robotic Defense System" is turned into an acronym it reads TARDS, a possible Doctor Who reference, or a play on a contraction of the word "retard".
In the third room, there are Banshees in the glass windows.
In the room with the Pelican thrusters lying around, the labels on them say "Abandon In Place."
Foundation is based on the Marathon 2 map, Thunderdome.
It is one of the very few Halo 2 multiplayer maps that does not spawn either a Sniper Rifle or a Beam Rifle.
If you are playing alone, there are numbered signs on the walls directing you to the respective labs.
If you look at the "pillar" in front of the rocket spawn from the sword spawn on either side, you will spot a symbol. The symbol is still a matter of debate. Some say it looks like the Halo 3 icon, others the Halo 2, a face, and some just a texture. It can also be found in the room with the Pelican thrusters next to the humming air vent.
There is a way to break the barrier and leave Foundation. In "spur room" #1, sword cancel at one of the fans in the ceiling holes, then, once 2 people are on the thin ledge that you should now be standing on, sword cancel toward the top of the hole. After you get out of the level, you can walk around the entire thing and out to where the map ends. If you get tired of the outside and want to go back into the interior of the map, just jump back in the fan-hole and you can get back into the boundaries.
There's a secret "fifth room" in Foundation. Here's how to get to it:

All players must be Elites.
Set the vehicles to random, and hope you get a Gauss Warthog and a Ghost.
Go to room 1, and shoot at the gray platform thing with the Gauss cannon until it is standing upright.
Position the gray thing UPRIGHT to the second crack in the wall with a friend.
Drive the Ghost to the right of this gray thing.
Carefully Drive the Warthog on top of the Ghost.
Have everyone get inside the ghost, then get out, one at a time.
You should be pushed into a small room.

It is possible to get out of the map if you butterfly in room 1 through the fan vent.
Foundation was recreated in an official Bungie made Forge map located on Sandbox known as Excavation.
Foundation is probably one of the most recreated maps, next to Warlock, using Bungie's dedicated Forge map editor.
Foundation was a popular map for the SWAT gametype.
The battle sounds that are heard on the level Outskirts are reused on the map.

Sources