The Fort of Santa Caterina of Porto Ercole is a coastal fortification located on Mount Argentario, near the homonymous center, on the slopes of a promontory that slopes directly towards the sea, dominating the Old Port from the east.
The fortification was built in the first half of the eighteenth century by the Spaniards, in the place where there was probably a pre-existing lost defensive structure; it became a place specialized in the attacks against possible enemy boats that approached this stretch of coast of the State of the Presidi.
Its construction, close to the Forte Philip, was intended for the control of the bay in which currently stands the port of Cala Galera, once it was decided the transformation of the nearby Torre del Mulinaccio into a windmill.
The functions of guard and offense characterized this structure until the beginning of the nineteenth century; with the annexation of the entire territory to the Grand Duchy of Tuscany, the fortification was progressively abandoned. At the end of the century, the structure played the role of an infirmary for the prisoners who were detained at Fort Filippo.
The fort Santa Caterina develops with an irregular pentagonal plan, with powerful and thick curtain walls, sometimes covered in stone and sometimes in whitewashed plaster, which incorporate on the side opposite the sea, a series of buildings, one of which has a summit terrace to perform the sighting functions.
The curtain walls have other openings (loopholes and windows), where the gunboats once were located. At each of the two corners facing the sea, there is a guard post with a semicircular section, without covering, resting on a protruding shelf in the shape of an inverted semicone.
Access to the building is through an arched door, which opens at the ravelin, above which a rectangular window opens; in the upper part there is a coat of arms of the State of the Presidios.
Inside the complex there was originally a powder keg, in addition to the rooms used for housing the military garrisons.