The Peloponnese has had a turbulent history, with the Romans, the Venetians, the Crusaders, the Ottomans and the Byzantines all occupying various different parts of the peninsula at various different times. Of course, they all had to defend their conquests against attackers, so they built castles and fortified enclosures at strategic locations in more or less every corner of the Peloponnese.  Many of these are still standing, and indeed several have been partially or fully restored so you can see them much as they were in their heyday.