The majestic huge edifice of the Venetian Castle rises above the island capital. It was built on the original ancient construction in the 13th century with the contribution of the Venetian emperor Markos Sanoudos II, and the use of some parts of the unfinished Temple of Apollo from which only a gateway named Portrara above the port has remained. Like other castles, the Venetian Castle was also the fortress protecting the town against raiders coming from the sea. Its four-storey defence tower of Glezos served as a watchtower, together with more towers scattered on the island, and at the same time as a medieval signalling device in case of danger. Inside the castle, there is also a museum and a cathedral.