Carthage plays by different rules than almost every other civilization in the game — restricted to a single City for its entire campaign, in exchange for turning every one of its Towns into a fully-featured trade hub.
In its Apex Antiquity Age, Carthage can only have one City. In exchange, its Towns can purchase Water Buildings regardless of their Town Focus (but cannot use Convert to City), and Carthage receives a second Merchant or Colonist unit every time it purchases or trains one. The single-City restriction actually persists into the Exploration and Modern Ages as well if you continue playing Carthage there.
The Numidian Cavalry is a unique Cavalry unit that can only be purchased with Gold, not trained normally, and gets increased Combat Strength for every Unique City Resource assigned to your Capital. The Colonist is a unique Settler that grants +1 Population if founded next to a Resource, plus extra Movement while Embarked.
The Punic Port is a Unique Quarter that lets Carthage purchase Water Buildings in every one of its Towns — including both buildings the Quarter needs — which increases Resource Capacity for the rest of the game.