At the top of the island of Agios Nikolaos, opposite Gaios, is the Venetian castle of the same name. The construction of the castle is said to have been completed in 1510 and is the work of the Venetian baron of the island, Adam II.

Most of the castle walls remain intact to this day, as do the Venetian tank, the gunpowder store, the watchtower and the cannons.

Access to the castle is by small boat from Gaios, but it could also be done by swimming, as the islet is less than fifty meters from Gaios.