The museum occupies the keep of the original barracks. It is a Grade II listed building and the ground floor preserves its previous layout. The museum is entered through the ammunition and powder store, and through this is the archway that used to lead to the rest of the barracks. Close to the archway are the guardroom and cells, one of which is equipped as it would have been when it housed prisoners. In the west tower is a rope winding gear to hoist weapons, ammunition and other stores to the upper floors.