The military construction, which dates from 1544, was erected under the reign of Emperor Charles I, as an essential complement to the Citadel. It has the shape of a five-pointed star, with pronounced angular corners for defence against enemy projectiles. It is built on a grand scale with two-metre thick walls. It had three terraces for batteries of cannons, howitzers and mortars, set at different heights, to defend the port and the coast, It had a garrison of up to two hundred men. The building remains an extraordinary example of coastal artillery fortress.