In 1587, Sir Francis Drake attacked and burned St. Augustine, resulting in the consolidation of Spanish forces in the area. Some years later, Castillo De San Marcos was built to help protect the area from invasion by the English at Charleston. The fort was built by slaves, soldiers and residents of the city. The outer walls, 12 feet thick at the base and tapering to 7 feet at the top, were built of coquina blocks, quarried on nearby Anastasia Island and ferried to the site. The cost of San Marcos Castle, as it was called, was estimated at millions of dollars, a sum that prompted the king of Spain to exclaim, "Its curtains and bastions must be made of solid silver."