Here's an artist's representation of the fabled city of Carthage. Cool stuff.
According to Wikipedia:
Two large, artificial harbors were built within the city, one for harboring the city's massive navy of 220 warships and the other for mercantile trade. A walled tower overlooked both harbours.
Carthage was one of the largest cities in Hellenistic times (by some estimates, only Alexandria was larger) and was among the largest cities in pre-industrial history.
The Carthaginian Republic was one of the longest-lived and largest states in the ancient Mediterranean. Reports relay several wars with Syracuse and finally, Rome, which eventually resulted in the defeat and destruction of Carthage in the third Punic war.