Page "Fortification" Paragraph 77

Fortifications designed to keep the inhabitants of a facility in rather than out can also be found, in prisons, concentration camps, and other such facilities, with supermaxes having some of the strongest of those.

Those are covered in other articles, as most prisons and concentration camps are not primarily military forts ( although forts, camps, and garrison towns have been used as prisons and / or concentration camps ; see Theresienstadt, Guantanamo Bay detention camp, and the Tower of London for examples ).