By the 16th century, the Ottoman Turks were fast advancing into Europe. Between 1573 and 1580, the Venetians built this enormous fortress, completed with sturdy bastions, to protect the island against Turkish invasion and also as a place where locals could take shelter, should the Turks take the town. Perched atop Paleokastro hill, immediately west of the old harbor, it affords marvelous views over Réthymnon old town and out to sea. On the highest point, note the mosque, originally a church, but converted into an Islamic place of worship by the Turks when they eventually conquered the town in 1646. There is also a small open-air theater, which hosts concerts in summer.