This is the Old Navarino Castle, also known as the "Paleokastro".

It's a castle built in 1278 by Franc crusaders and is still standing on top of the hill, overlooking the lake of Divari, Voidokilia beach, the islet of Sfaktiria and the vastness of the Ionian Sea. Right before the castle, you get to catch your breath in this huge cave called "Spilia tou Nestora". If you're doing this hike in the summer, make sure you've got plenty of water with you and a hat, as it can get unbearably hot.

Some of the surrounding sights and spots you should not miss are: Voidokilia beach, the Archaeological Site of Ancient Messene, the Neda Waterfalls, the city of Kalamata, the fortified castle of Methoni and the beach of Finikounda.