Between the sixteenth and seventeenth century, the Crown of Spain built the towers of San Giovanni (St. Johns), of Torre Vecchia or San Marco (Old Tower or St. Mark) and the Sevo (Turr’e Seu) along the coast of the Sinis of Cabras. The towers were part of a coastal defense system against pirate raids from North Africa, built to protect the local population. Another tower called “Su Pottu” was probably erected at the end of the seventeenth century on the banks of the pond of Cabras with the task of guarding and protecting fishing activities in the lake basin.

The tower of San Giovanni, so named for being close to the church of San Giovanni di Sinis, was built between the late sixteenth and early seventeenth century on top of the hill (50 m above sea level) which dominates the archaeological area of Tharros. The size of this tower is considerable, with a widely view overlooking the Gulf of Oristano and offshore; it was armed with cannons and spingardes, and manned by a garrison of an alcaide (alcalde), an artilleryman and four soldiers. It is thought that the tower was built on the ruins of a nuraghe and a Punic tower, with stones reutilized from the city of Tharros. It consists of two superimposed cylindrical structures with a base diameter of 14 m and a total height of 15 m. The entrance is placed about 8 m above the ground, accessible today via an external staircase built in the nineteenth century; a large circular room with a vaulted dome, lighted from above through a skylight, has got a trap door on the floor that allowed access to the cistern used for collecting rainwater; a fireplace and the “santabarbara” (munitions depot) are placed around it. In the second half of the nineteenth century, when the tower was manned to crack down on smuggling, two apartments were built on the terrace that was accessible via an internal staircase. The building was restored between 1987 and 1990.

The other tower built to protect the area, called Torre Vecchia or tower of San Marco, is located along the eastern coast of Capo S. Marco. It is a truncated cone-shaped tower with a base diameter of 7.55 m, a diameter of 7.25 m at the top, and a height of 8.70 m. Access to the tower occurred directly to the terrace by means of a ladder or a rope ladder, which during emergencies could be easily removed by the security personnel. The tower is constructed with sandstone blocks and it originally had masonry walls plastered with lime mortar.