From Halopedia, the Halo wiki
"Narrows", is a fully symmetrical map, with two opposing bases on each end connected by a Forerunner Causeway. The weapons in each base are identical, with a few minor changes. A man cannon exists on one side of each base, and propels the player to the opposing base on the other side of the canyon. If players are to enter opposing man cannons at roughly at the same time they will collide in mid-air and both fall to their death. The main bridge features two different levels for players to combat each other. The upper level is vaster, allowing players to be hit from multiple sides and angles. The lower level, connected by a "lean" light bridge is more discreet, and often less hazardous.
Each team spawns at opposite sides of the map, and must cross the bridge or jump through the man cannon to engage the enemy team. The two bases have been nicknamed "Blue Base" and "Yellow Base" by players, due to the blue and yellow tinting of each base. From Blue Base, players can access the man cannon to their right, while Yellow Base players will find the man cannon to their left.
Each base features identical weapon spawns and placement. The main differences in each base are the aesthetics, equipment, and power-ups. Yellow Base players have a power drain placed in front of their man cannon, and Blue Base players have a bubble shield. Each base features a low point, which is even lower than the "Low Bridge." Blue Base players have an overshield power-up available, and Yellow Base had active camouflage, though it was replaced with an overshield in an update. In addition, this low point contains a Brute Shot (available on both sides).
This bridge connects the two bases, and is generally the area where most of the fighting occurs. The upper bridge features a lot of open areas and not a lot of room for cover. The rocket launcher used to be found at the center of this level, but this was replaced by a bubble shield. Separated from the top, but accessible via ramps on the four corners of the bridge, you can find the lower bridge.
This lower bridge offers more protection than the upper level. A rocket launcher replaced the shotgun spawn in the center of the light bridge, which connects the two bases. Players can also find the sniper rifle on each side of the bridge. Taking the lower bridge may be a safer and quicker route to each base, but can leave the player vulnerable to rocket or sniper fire. It is also possible to fall off the bridge and land on the cooling core below, though you will be killed by The Guardians after 5 seconds. Careful players can jump down onto a covered ledge and hide on this cooling core without dying.
The map's design is similar to the outdoors bridges on the Halo: Combat Evolved campaign levels Assault on the Control Room, Two Betrayals and the multiplayer map Gephyrophobia, although Gephyrophobia was noticeably bigger. It also seems to borrow elements from the Halo: CE map, Longest.
As of the December 11, 2007 update, two of the Maulers that spawned along the top of the bridge (one on each side) have been removed. The remaining Mauler on each side was re-positioned centrally and slightly back from the original locations, behind a wall offering cover from the rocket launcher spawn location. In addition, the radar jammer has also been removed, though this happened in an earlier Matchmaking update.
It is believed that this change was caused by near-constant complaints regarding the large number of Maulers in many levels combined with the Mauler's ability to nearly-instantly kill an opponent at very close range. Many people felt that the weapon was unbalanced because of the close-quarters power of the Mauler, especially in CQB-style levels.
As of August 22, 2008, the shotgun and the active camo were also removed and the rocket launcher has been moved to the location where the shotgun was. The bubble shield is now on the top where the rocket launcher was, and there are two overshields in the same location.
A remake of Narrows was concepted for an unreleased Halo 4 map pack, which would have redecorated the map with a human aesthetic and a Pelican in the background, as opposed to the Forerunner style of this map. Other maps in the unreleased map pack would have included "Arrowhead" and "ONI Bog". The map was pitched to 343 Industries but was ultimately never produced.
Halo 4 Concept art