From Halopedia, the Halo wiki
Fort Deen is a symmetrical map. It has three starting bases for the player and his/her allies on either side of the map. It also has four additional bases, each in close proximity to the starting bases, which are initially occupied by rebel bases and defended by rebels. In the middle of the map, there are two energy walls, which, when garrisoned by infantry, activate impassable energy barriers, blocking any direct access to the other side of the map. There are, however, side-paths on each side of the map that can be used as a secondary route, but they also have garrisonable energy walls. If all routes to the other side of the map are blocked by energy barriers, there are a teleporters on each side that can transport the units to the other side of the map, beyond the energy walls, allowing quick access to the enemy's side of the map. Both teleporters have deployable lookout towers stationed next to them.