At the heart of the castle is the 16th-century Henrician fortification. Built of a combination of granite ashlar and rubble, it comprises a circular keep surrounded by a gun platform, entered through a bridge and a forebuilding. The keep has thick walls and on in the inside is octagonal. The basement was originally a kitchen, cellar and larder for the castle. The ground floor was initially designed to be a gun room, complete with gun embrasures, but was altered during the initial construction project to form living accommodation for the garrison instead. Another gun room occupies the first floor, with seven gun embrasures, and is dressed to appear as it would have done in the 16th century. The roof has seven gun embrasures and a lookout turret.