This is Feraklos castle. It was built during the Byzantine era and was captured by the knights in the early 1300s as their first foothold on the island. They started repairing it about a century later and considerably fortified it. Lindos, Feraklos and Rhodes itself were the three strongholds of the knights on the island. Feraklos was captured after a long siege by the Ottomans in 1523. Rhodes fell first.