Forge is the name of the map editor that Bungie delivered with Marathon Infinity. Forge is available at The Trilogy Release Page, and runs on Mac OS 7.3.1 to 9, although it runs best on OS 7.5.5 and later. Forge can also be run through Mac OS emulators such as Basilisk or Sheepshaver or through running Classic, on later non-intel based Macs.
Forge uses a 2D draw mode to design maps, add items, scenery, monsters and players, and also to set polygon attributes, such as liquids, platforms, monster/item triggers etc. The texturing and lighting on a map is done in a 3D "Visual Mode" , where the player can move around the map as if in-game. Some things such as polygon heights and floor and ceiling textures/lights can be done in both modes, and it is up to the map makers discretion which to use.