From Halopedia, the Halo wiki
For bases in fictional context, see military base.
Usage in multiplayer
Usage of bases usually varies depending on a mission or multiplayer gametype. Usually, a base is used to provide protection, or contain stronger weapons or equipment.
Throughout the Halo series, these bases may contain:
Although bases are mostly used for CTF, they can be particularly useful in Oddball (to hide in, if nav points are disabled) and King of the Hill. Bases range from small rooms all the way to a huge tower with mini buildings. For example, in the map Infinity, one team also has a sub-base, quarter way to their destination.
The only map in which both bases are literally symmetrical -- including the whole map and the level geometry -- is Rat's Nest. If exact geometric symmetry isn't required for bases to be considered "symmetrical", however, then many other maps have symmetrical bases.