The Fort of Santa Caterina in Porto Ercole, on Monte Argentario, stands on the slopes of a promontory that drops steeply down to the sea, overlooking the Old Harbour to the east. The fortification was built in the first half of the 18th century by the Spanish, on the site where a pre-existing defensive structure, now lost, probably once stood; it became a base specialising in attacks against any enemy vessels approaching this stretch of coastline belonging to the State of the Presidi.

It was built near Fort Filippo to control the inlet where the port of Cala Galera now stands, following the decision to convert the nearby Mulinaccio tower into a windmill. This structure served as a guard and defensive post until the early 19th century; with the annexation of the entire territory to the Grand Duchy of Tuscany, the fortification was gradually decommissioned. By the end of the century, the structure served as an infirmary for prisoners held at Fort Filippo. Fort Santa Caterina has an irregular pentagonal layout, with massive, thick scarp walls, partly clad in stone and partly in roughcast, which, on the side facing away from the sea, incorporate a series of buildings, one of which is equipped with a summit terrace for use as a lookout post.

The curtain walls feature further openings (loopholes and windows), where embrasures were once situated. At each of the two corners facing the sea, there is an open-air, semi-circular sentry post, resting on a projecting corbel in the shape of an inverted cone. Access to the building is via an arched doorway, which opens onto the ravelin, above which there is a rectangular window; at the top is a coat of arms of the State of the Presidi. Inside the complex there was originally also a powder magazine, in addition to the rooms used as accommodation for the military garrisons.