The major evidence of the existence of defensive city walls are preserved to the north of the city, on the hill of Murru Mannu and on the northern slopes of the hill of San Giovanni.
In Su Murru Mannu the most impressive remains of a fortified complex consist of a curtain wall, a moat with a counterscarp, and a rampart.
There is also an inner boundary formed on the sides facing the moat by a wall of large polygonal blocks of basalt, interspersed with sandstone ashlars or blocks. In the western section, a basalt face wall was added later to the double face wall made by large blocks of sandstone. The wall includes two small back gates (posterns), one of which was constructed entirely of sandstone blocks fit perfectly into the basalt facing, while the second is preserved only in segments of the base. The moat, which was approximately 10 meters wide, had an inside counterscarp (embankment) lined with large polygonal basalt boulders, which would have made traversing the moat much more difficult.
The fortification was later converted to a Roman necropolis, apparently after its ruin in the 1st century BC. It is now thought that walls of the fortified structures made of basalt blocks were constructed in the Republican age (2nd century BC), while the walls made of sandstone blocks were part of a Punic fortification, built between the late 6th and 5th centuries BC.
On the slopes of the hill of S. Giovanni, there was discovered a stretch of defensive walls and a tower constructed of large square blocks of sandstone. These walls have a double face structure and descend along the northern slope of the hill in a zigzag pattern.
The tower consists of a quadrilateral base made of large square blocks, in origin joined together possibly with wooden cramps, on which a curvilinear wall, which has been partially preserved, was placed.
Some scholars date the tower back to the Carthaginian age, because of the presence of several Punic type battlements (with curved tops); while others date it to the Roman and Byzantine times. Other sandstone blocks present at the foot of the tower may have been part of a Punic fortification.