Citadel

The Citadel is a colossal deep-space station in the Mass Effect Universe. Strategically located at the junction of a number of mass relays leading to various parts of the galaxy, it was a hub of activity for spacefaring species.

The Citadel is a massive construct, similar in shape to a pentagram. It consists of a central ring 7.2 kilometers in diameter, from which five arms protrude, each 43.6 km long and 330m thick. In addition to the many layers of defence it has, the station is capable of closing in on itself, via the convergence of its five arms, transforming it into a long, impenetrable cylinder.

The Citadel was not constructed by the Protheans. It, and the mass relays, are the creations of the Reapers, and were built to help facilitate their cycle of galactic genocide. The station is actually an inactive mass relay leading to intergalactic (dark) space from where the Reapers came.