The Castillo de San Marco protected the city of St. Augustine from attack via the St. Augustine inlet, but could not protect from ships approaching from the south along the Matanzas River. Fort Matanzas was built to protect this "backdoor" to St. Augustine. The five cannons at Fort Matanzas could reach the Matanzas inlet, which was only a half mile away when the fort was built in 1742. The shorelines have changed dramatically since that time. The entire fort is 50 feet by 50 feet and 30 feet tall. It was constructed of coquina stone. Today, four cannons are located on the deck. Two are originals dating from around 1750 and two are modern reproductions. The sentry box (garita) fell off the structure sometime in the 1800s, but was rebuilt of coquina in 1929. The door from the gundeck leads to the soldier's quarters and the stairs lead up to the officer's quarters.