A standard Martello was 12-15 m (36-45 ft) in diameter, some 12m (40 ft) in height, with thick walls of heavy masonry. The interior was divided into three stories. The ground floor was the storerooms for ammunition and provisions. The first floor had several rooms, with fireplaces built into the walls for cooking and heating, and was intended for a garrison of 24 men and one officer. The roof was mounted with a 24-pounder cannon, able to rotate 360°. A well or cistern supplied the garrison with fresh water. A tower had a single doorway 5m (16 ft) off the ground that could only be reached by a removable ladder. A few were surrounded by a moat.