The Citadel was designed to act as a last defence should the remainder of the Western Heights defences fall to enemy attack. Its design consists of three redans to its western side and a single redan to the eastern flank.  There are traces of gun positions around the Citadel, as well as magazines and barracks.

By far the most impressive is the Officers’ Mess, pictured above.  The excellent condition of the Citadel is due to the fact that the Home Office has been in possession of this site since the 1960s, and it has seen use as a Youth Offenders Institution and, more recently, as an Immigration Centre.  This has given rise to modern buildings being built on the site, but these do not detract greatly from the main Citadel, and it is still clear how impregnable this fortress was designed to be.