The Punic Ports are the most evocative surviving Punic-era structures at Carthage — a pair of artificial harbours hand-dug by the city's engineers and connected by a narrow channel to the Mediterranean. The southern port was rectangular and used for commercial shipping; the northern one, perfectly circular and 130 metres across, was the military naval base — at its peak it could berth 220 warships in covered dry docks ringed around an admiralty island.
Today the surrounding land has been built up around the harbours, which look small and ordinary at first glance. The circular shape of the military port is clear from above (Google Maps shows it perfectly) but from ground level the magic is in the small museum and the visible stone foundations of the individual dock bays. Combine with a visit to the Tophet just south, and the Antonine Baths to the north.