The fort, on Rattlesnake Island, consists of a square with walls 50 feet (15 metres) to the side, topped by a tower 30 feet (9 metres) high. It is near the site of the slaughter of some 300 French Huguenots by Spaniards under Pedro Menéndez de Avilés (Menéndez de Avilés, Pedro) in 1565, and its name is the Spanish word for “slaughters” or “massacre.” In 1569 a wooden watchtower was built at Matanzas Inlet, at the southern end of the inland waterway, to guard St. Augustine from approaching ships. The tower needed to be replaced frequently because of the region's warm, wet climate. In 1740 a British siege of St. Augustine convinced the Spanish that a stronger fortification was needed, and the fort, built of coquina (shell stone), was begun; it was finished in 1742. The fort was later ceded to the British (1763–83), briefly returned to Spain, and finally handed over to the United States in a treaty of 1819. After U.S. troops took possession in 1821, the fort was not occupied and began to fall into ruin; restoration work began on it in the 1920s after it became a national monument. The surrounding parkland is home to sea turtles, crabs, and a variety of birds, including the least tern and wood stork.