The Byzantine Castle on the island of Simi was also the site of the Knights Castle, built by the Knights of Saint John in the 14th century. The Knights Castle shared a wall with the ruins of the Byzantine Castle, many of which are still visible. The castle also shares the wall that surrounds the Acropolis, on top of the hill.
The castle contains an emblem of the Knights of Saint John over the main entrance and is located at the highest point of the Upper Town. It overlooks the harbor on one side and a small valley on the other, offering a breathtaking view of the entire town as well as of the Aegean Sea beyond it. There is also a small chapel dedicated to the Virgin Mary (Kyra Panagia). This chapel occupies the highest point on the castle and offers spectacular views of the towns of Horio and Yialos.