A small lighthouse was erected between the two harbours, to the north of the channel that connected the two basins, at the west end of a short internal mole. It belongs to the period of Trajan and was restored in late antiquity. The original structure had a rectangular plan, with the long side (21.80 metres) facing west. Waves could pass through holes in the base. High up there was some sort of balcony supported by travertine consoles.
In the third or fourth century the structure was enveloped by another wall and restored. This lighthouse seems to be depicted on the Tabula Peutingeriana.