These ruins are of ancient baths there. The balls shown in the second photo are canonballs from the Punic Wars (The term Punic comes from the Latin word Punicus (or Poenicus), meaning "Carthaginian"). The Battle of Carthage (assume that this is the last one when Rome won) is mentioned in the movie "Gladiator." Below is a description of the ruins from the UNESCO website:
Carthage was founded in the 9th century B.C. on the Gulf of Tunis. From the 6th century onwards, it developed into a great trading empire covering much of the Mediterranean and was home to a brilliant civilization. In the course of the long Punic wars, Carthage occupied territories belonging to Rome, which finally destroyed its rival in 146 B.C. A second – Roman – Carthage was then established on the ruins of the first.