From Halopedia, the Halo wiki
During the Covenant’s first assault on Earth and the occupation which would follow, Old Mombasa became a key engagement point for UNSC forces seeking to prevent the Covenant from establishing what they initially believed to be an invasion beachhead on the planet. Recreated in the War Games environment to train Spartans for urban warfare, District is a near-perfect simulation of the terrain as of October 20, 2552.
The map's layout includes a portion of geometry from the mission Outskirts, with the starting area, courtyard and more accessible and several alleys and roads un-blocked. Like most Halo 2 maps, District provides certain landmarks that help players. Such landmarks include the atrium; the Voi Theater, and the three level parking garage, as well as many small shops.
This map was also created for larger scale battles. It has "about two-thirds the size of the playable area in Coagulation, or, it offers about four times as much playable area as Turf." It also includes many vehicle accessible places such as elevated sidewalks, a small bridge, and ramps all around the map. With all of its high places, it makes it a perfect map for snipers.
District also has a "mix of close-quarters and wide-open combat spaces". Many of the areas are closed but are "still open to the sky, allowing players to see the surrounding building tops and cityscape".
"District was designed from the very start to focus heavily on vehicles and on creating fun and interesting gameplay situations between vehicles and infantry."
Differences from Outskirts
District shares geometry with several areas of the mission Outskirts, from the Halo 2 campaign. The courtyard area in which John-117 and his squad of Marines defend against waves of Covenant infantry serves as a focal point of the map. Unlike Outskirts, however, there are several differences accounting for multiplayer map design. The gate at the end of the courtyard, where the Yanme'e attack from in the mission, is now open and the road follows through, up and around to the left, connecting to the overpass section later on in the mission where Johnson boards the Pelican.
The gate where the Hunters spawn is also open, and is largely similar in layout, including the ledge to the right where the Sniper Rifle spawns in the mission. Unlike Outskirts, however, this ledge does not end in an alley but instead follows through to the aforementioned road. The overpass itself, which in Outskirts is destroyed, is fully intact on District, and follows round and down, connecting to the road in the Outskirts courtyard where the NMPD building is in Halo 2: Anniversary remastered graphics. The most drastic changes come in the form of the area where the Jackals spawn in Outskirts, however, and the alley leading into Sniper Alley. The building to the right of this alley has been replaced with a multi-level parking lot, and the alley largely restructured so that no trace of the passage leading to Sniper alley remains.