Also known as the Tower of Olives, it is one of 104 that guard the coastline of the whole of Sardinia.
Its erection, dating from around 1578, was due to the viceroy of Sardinia Don Miguel De Moncada.
These were the years immediately following the Battle of Lepanto (1571), when the Iberian crown, in order to counter Islamic piracy, now considered a passive defense system, based on fortifications, more economical than the active one, with the use of large military fleets, which had been preferred until then.
The tower of Su Loi was of the smaller type, called a torrezilla, that is, a simple lookout point placed 10 meters above sea level, manned by just two men.
It was meant to ensure visual continuity between the towers dotting the entire Gulf of Cagliari and to guard the nearby mouth of the rio San Girolamo, to prevent the enemy from obtaining drinking water.
Its structure, of a truncated cone shape (base diameter 6.40 m; height 8 m), rests on a base reinforced on the inside by two cross-intersecting buttresses.
The practicable floor consisted of a wooden loft supported by a continuous fold of the wall.
It was accessed through a hatch located 3.50 m above the ground, via a rope ladder.
Through a trapdoor opened in the lowered domed vault one ascended to the terrace, enclosed by a high parapet interspersed with wide slits.
From it, two sentry boxes stretched inland: the first, mounted on sandstone corbels, protected a machicolation for the launching of lead bullets, placed at perpendicular to the entrance hatch; the second, supported by two juniper trunks, was intended to house the ammunition depot, so that, as usual, any explosion of it would not affect the stability of the entire structure.
The Su Loi tower, which depended on the alcalde of Sarrok’s Ballast Tower, remained in use until 1843.