Castle Williams was built from 1807 through 1811 on the northwestern corner of the island, on what was then a submerged rock. Named for USACE chief engineer Jonathan Williams, it is a cylindrical four-tiered sandstone building measuring high by in diameter. The walls taper from from bottom to top. The building is four-tiered, with 13 casemates on each tier each having a capacity of two cannons, for a total capacity of 104 cannons. Two structures inside the southern side of the fort were removed in 1900.