A few metres from it lie the ruins of an older tower, dating back to the Spanish era and collapsed due to erosion. Cala d'Ostia was designed by engineer Daristo and the work carried out by master builder Vincenzo Cara.

Completed in 1777, the building was erected about 10 metres away from the Spanish tower. Further maintenance work is documented in 1784 and again in 1808.

The tower was built with squared blocks of local sandstone and rounded granite pebbles, with a 13-metre-tall cone-shaped structure. The entrance opens about 5 metres from the ground and was protected by a sentry box, of which the wooden brackets are still visible.

Inside, a barrel-vaulted chamber with a stairwell built into the wall is lit by a single slit. The upper terrace has battlements for the insertion of rifles and in the part facing the sea; the parapet is lowered to contain three gun ports. It was a tower 'de armas' and not a simple lookout, so it housed a garrison with a captain, a gunner and three soldiers. Near the building, the remains of a small lime kiln can be seen. The tower was abandoned in the second half of the 19^th century.