The seat of power in the Covenant, the Citadel is the main nerve center for the interplanetary warfare of the Covenant. It offers a modular system that makes aggressive colonization and rapid deployment of troops and armor effective. This is where the political and religious leaders of the Covenant or their underlings issue orders to the various races and cultures that create the Covenant.
Outpost
Building Class: Outpost
Units Trained: Scarab, Suicide Grunts (and upgrades; Arbiter only), Brute Infantry (and upgrades; Brute Chieftain only), Elite Honor Guard (and upgrades; Prophet of Regret only)
Use this to construct a new outpost base. Eventually, it can be upgraded to a full Citadel, increasing your ability to crush the evil enemy. When you destroy a neutral or enemy base or when you locate an empty base pad, you can build an Outpost, providing you have the supplies. It has three building pads and doesn’t feature any Turret base slots. Whether this is a good idea depends on the map, as some base locations are better than others; how strong the enemy is; and what position your forces are holding. Refer to the Campaign, Skirmish, and Multiplayer tactics for all the available base locations.
Keep
Building Class: Keep
Upgrades base to a Keep, which adds two building sites. Unlike the UNSC leaders, the Covenant’s heroes all upgrade their Outposts without any advantages over each other. The Keep has five expansion pads and four Turret base slots. If you fear enemy attacks, it is better to upgrade from an Outpost to a Keep and augment it with Turrets. It is wise to time the upgrade of this base to a Citadel so that the Citadel is built just as the last building rises up from a Keep.
Citadel
Building Class: Citadel
Upgrades base to a Citadel, which adds two building sites. Each building adds to the strength of the central structure. It has a total of seven expansion pads and four Turret base slots; these aren’t additional but simply add two more building pads to a Keep. Judge whether you need the two additional pads immediately—they are certainly needed on your initial base. It is almost always wise to upgrade to a Citadel, as the buildings allow more damage for the central structure to absorb.
Other Base Functions
Base Rally Point
New units from this base will move to this location when training is complete. Use the Global Rally Point to override this. Using this flag option is extremely important, as it allows you to point to an area of your map and have any troop type head there after building or training; this effectively takes the manual labor out of this function. Remember where your Rally Point is, in case it falls to the enemy—you don’t want to send your troops straight into danger! Every base has its own Rally Point, allowing you to mass separate armies if you want to, say, tackle a base from two directions.
Self-Destruct
Instantly destroy this entire base to free it up for an ally. This is really the only fortuitous method of using this function; otherwise, employ this if your base is under attack so your foe doesn’t have the satisfaction of destroying your base with his forces. You gain Supply Points if you blow up your own base, but do this only if you have troops outside or elsewhere on the map that can quickly locate and build a new base. You are still at a serious disadvantage.
Lock
Units train as normal but are garrisoned inside the base instead of exiting into the world. Unlock will release all the garrisoned units at once. Locking and Unlocking your base allows you to surprise a foe who is checking out your location and allows you to keep troops that you are massing safely inside the protective walls of your base. When you Unlock or Lock your base, the ramped doors clamp open or shut, allowing you to know whether an enemy’s base is open. When the doors are closed, this means you or your foe are massing troops. However, if you get into the habit of always Locking your base, you can fool your foe into thinking you’re massing troops, whether you are or not! Use this secrecy to your tactical advantage.