Forge

Forge is a Halo 3 gameplay mode that allows for both playing and designing multiplayer maps.

Background
A multiplayer focused tool, Forge will allow players to customize multiplayer maps, changing and resetting spawn points, weapon spawns, weapon timers, vehicle spawns and timers, vehicle spawn points, and add existing geometry to the map’s existing structure in the form of crates, boxes, fusion coils, lifts, teleporters, AIE-486H Heavy Machine Guns, Plasma Cannons, both on their tripods, Ball Turret and more.

The amount of items you can drop is regulated by an economy budget system, with different items having different values and each map having an overall budget that the items you place cannot exceed. By deleting existing things off of a given map, you can actually place more things. Players can modify and create their versions of maps, save them and share them with their friends, but that’s only half or so of what Forge allows.

In addition to functioning as creation-driven tool, Forge is also a playspace. Up to eight players can be on a given map at a time, shifting back and forth between their chosen multiplayer model and the Forge model – a Monitor. Players can play actual games in Forge, goof around, kill each other, play Slayer where each team designates a Monitor to supply weapons, vehicles, equipment and cover on the fly.

Throughout the time you're editing you can press up on the D-Pad and change to your multiplayer model and play.

Once you're done with the editing you can start a party over System Link or Xbox Live and see how it works. You can build crate towers, grab a vehicle underneath a teammate, or have a friend jump on a crate and move it throughout the map.

Other things forge allows is the other players can join fun! For example, lets say your friend is building a city of fusion cores. You (as the Player) can lug a grenade at his pile and destroy all his work! Boom! Then your friend will switch to a Spartan and destroy you. Good luck!

Apparently, all items spawned on forge will have a price, with weapons, crates and smaller items being cheaper than vehicles. There is a spending limit to the map which restricts total items than can be spawned.

Trivia

 * It has been stated that the Elephant will not be among the objects that can be spawned due to its size and specific purpose on maps.
 * Forge will keep track of stats and kills. It has been confirmed by Bungie that you can save your creations. Also, if your creation is liked by Bungie, they will put it up to be downloaded by other players for use.