The Citadel was the heart of Dover's Western Heights fortress. Construction began at the end of the 18th / beginning of the 19th century, building on earlier earthworks. It was designed so that each ditch is flanked by gunrooms and is an angular shape reminiscent of the forts designed by Vauban. It was protected from the North by the Outer Bastion and counterscarp galleries. The Southern end is occupied by the casemated Officers' Quarters, which is designed in a mock Tudor style, and remains in good condition. Modern buildings now occupy the parade ground in the centre and few original structures remain on the surface. The original gatehouse and bridge are still used as the main entrance. It remained in army hands until the 1950s and has since been a Borstal, Youth Offenders Institute and more recently an Immigration Removal Centre. This has meant that it is free from vandalism, unlike the rest of the components of the Western Heights fortress, although a number of parts have been demolished to make room for new buildings.
The Western Outworks occupies the Western most part of the Heights' defences and is joined to the Citadel by a bridge. Barrack huts on the surface remain in good condition, both internally and externally and are within the DIRC perimeter. Outside of the perimeter, a row of Haxo casemates with Unmarried Soldiers' Quarters behind and two large sets of casemated barracks at ditch level are in a dilapidated state, and a large double caponnier has been buried with no current access.
There is no public access to the Citadel, but I worked there some years ago, and was fortunate enough to be able to access some of the main parts.
The Outer Bastion.
Update May 2011: I attended a visit with the Fortress Study Group to the Citadel tunnels, including some recently opened parts of the Outer Bastion and the Counterscarp galleries. The tunnels are mainly in excellent condition, with some original doors remaining in situ. Unfortunately, due to security restrictions, cameras weren't allowed.
Aerial view, kindly supplied by Geoffrey Hall
The Citadel's Outer, Centre and Inner Bastions
Plan of the Haxo Casemates & two-tier South Casemates under
One of the Casemates , the floor between has been removed
Drain linking the South Casemates to the North Casemates