The principal tower, or gatehouse, is rectangular in plan by , and almost high, with a projecting round tower on the north-east corner, beside the entrance. It comprises the Lord's Hall, and three storeys of chambers above, located over the entrance passage. The vaulted, cobbled passage, long, was formerly defended by two sets of timber doors, and a yett, or hinged iron grille, remains. Guardrooms on either side overlook the passage via gunloops, and also on the ground floor is a well, in the basement of the round tower.