Foundry was part of the heroic map pack for halo 3. The map is essentially a big room filled with crates boxes and obviously weapons. the map usually plays quite well, with a rush to rockets or to the snipers on each end of the map, one of the faults however is that there are no power weapons near the spawn points making it very easy for one team to control the map.
Although the map is fun to play the best part is when your in forge. In forge you van delete everything on the map and be left with a huge cavern, you can then fill it with tons of new objects including mancannons and crates, the whole idea of the map is to be creative and some of the maps made by MLG are now used in matchmaking.
This potential for customization has made Foundry a very unique map in the Halo 3 experience, and a key tool for those that forge their own maps. By restructuring the layout of the crates, stairs, and walls players are virtually able to create any time of map they desire. This has led to a strand of Foundry maps created by players that have been implemented into matchmaking in various playlists. In addition to the preset tools Bungie has offered the players, forgers have found glitches in the forge and the map giving them the ability to merge objects together and have unlimited resources, which has contributed to the variety offered by Foundry.