It is a walled town and has a fine harbor defended by forts. In the time of Philip II of Spain, it was the great naval seaport of Spain.
It was founded about 230 BC by Carthaginian General Hasdrubal as Carthago Nova (New Carthage, which is what Cartagena also means.) When it was taken by Scipio in 210 BC it was said to be one of the richest cities in the world.
Hannibal got silver from the mines there to carry on the war against Rome.
Cartagena was the birthplace of Isidore of Seville.
See also Cartagena de Indias, a city in Colombia.