The Castle of Assos is an old Venetian fortress that has been left over from the Italian occupation of the island. Today, it is a beautiful pile of ruins that tourists can visit and learn about the history of Greece. You can hire a guy to take you around the different parts and explain the history, but I don't really recommend them as most of the information is available online if you're willing to do your own research. Please let me know if you've ever been! I would love to learn about what you thought of the castle.