From Halopedia, the Halo wiki
A Box Fort, as the name implies, is using crates, and loose objects found in many Halo 2 multiplayer maps and placing them strategically so that you may have the advantage over your enemies. It is best used on the map Foundation on the game types Zombie and CTF.
In Halo 3 using Forge mode, it is possible and achievable to make even more elaborate Box Forts and bases. An example would be using crates on High Ground to completely block off the base to the outside environment. The new Forge method that was added in Halo 3 also makes box forts easier, as you practically have an endless supply of them in your inventory. You can also make a sniper tower out of it by stacking them up and put a grav-lift on the back of one.
In the Grifball playlist, the boxes are used to separate the arena from the rest of the map.
In the Legendary Map Pack, especially on the map Avalanche it is possible to create box castles, or even giant robots made of boxes and crates.
On the map "Sandbox," even larger bases can be constructed, now in a massive "Sky Bubble," perfect for sniping on the map below.