Just after you pass the 10km marker when heading east on the PM-820 is a right hand turn into the remains of a Roman fort, Castellum Roma, that dates back to the third century. The sandstone foundations of the perimeter wall are clear, as are the bases of five towers.
Castellum Roma is a fortress-type building that was discovered in 1979 and excavated in 1979 and 1980. It is a quadrangular structure of 40m x 40m, with five rectangular towers, two of which guard what must have been the entrance gate. It is thought that it was never actually used, owing to the fact that neither structures nor ceramic remains have been found inside. The purpose of the fortress would have been to look out for boats coming into Es Calo, and to guard Cami Roma, which was the key road on the island at the time.
It has a similar plan to fortress constructions that defended the borders of the Empire in the Dabube, in the East, and in North Africa, which indicates that it could be dated to the Early Roman Empire. However, the exact period cannot be determined, as between the 3rd century and the Saracen invasion the Mediterranean was constantly vulnerable to attack.