Probably built around 1560 after the formation of the State of Presidi (1557), the Fortress is a remarkable example of Spanish military architecture. Sited on a hilltop in defence of the port, it has a square plan with barbicans, and rests on a square base formed by two terraces crowned with a series of machicolations. The stone wheels that made the cannon carriages rotate in the 19th century are still visible on the lower terrace. The base houses the warehouses, tanks and quarters for the garrison. Two exhibition sections are currently set up inside the Fortress: one dedicated to the archaeological finds collected on the seafloors of Argentario and Giglio Island; while the other documents the activities of shipwrights, that is shipbuilders.