From Halopedia, the Halo wiki
Gephyrophobia is a multiplayer level originally exclusive to Halo: Combat Evolved for PC and Halo Mac. It is included in The Master Chief Collection. The map bears some similarities to the Halo: Combat Evolved campaign level Assault on the Control Room. It consists of a large two-level metal bridge expanding over a misty pit, with platforms dug into the canyon walls on either side.
This map consists of three levels of play area: the bottom floor of the bridge, the upper floor of the bridge and the two ledges on the side of the canyon overlooking the bridge (connected by teleporters which is accessible in each base). In the middle of this map is the main support tower for the bridge. By default, the map provides an array of Ghosts, Warthogs and Banshees on each of these levels to maneuver around the map. Health packs are plentiful, placed strategically around the map to replenish player's health as they fight their way to the opposing team's base.
The level emphasizes control of long-range and power weapons. For example, sniping and the use of rocket launcher and fuel rods are quite common to repel players and eliminate enemy vehicles. Snipers often make use of the ledges to navigate around and pick their targets. Shotguns are often used in close-quarters in an engagement inside a base.