Two converging piers, now submerged, enclose the western harbour of Iasos. Given the position of the city set back from the open sea, and because of the wind pattern, their function was not to act as breakwaters, but to regulate naval passage. Their construction may date to the first imperial Roman age, when some nautical structures were built both at the level of the agora and along the eastern coast.

Probably in the mid-Byzantine period an almost square watch and defence tower was erected on the eastern and shorter jetty.

The dating of this building is still controversial. On the one hand, comparison with some castles in Cyprus, which have the same types of buttresses, and some historical considerations would suggest a construction date during a troubled period in Byzantine history, that is the 12th –13th centuries, when other defensive systems on this coast were strengthened to stand up to the threats from the sea. On the other, the Ottoman portulan of Pirî Re’is mentions a “bastion” at Iasos, constructed by Sultan Beyazid II Han to defend the “artificial port” between 1481 and 1512, which some have identified as this tower.