The Sea of Kythera is the western gateway to the Aegean Sea. It is often rough, which makes crossing it both difficult and dangerous. Specifically, Cavo Maleas, the southeast cape of Peloponnese, is one of the most dangerous in Greece. This is why it was necessary to have a harbor in Kythera to allow ships to find shelter in case of a rough sea. The port of Kastri, in the gulf of the former city of Scandia, and the one in Agios Nikolaos, both served as safe ports in the Antiquity.
West of the gulf of Avlemonas is a Venetian castle called Castello, the fortified castle of Agios Frangiskos. It was erected in 1565 by the Venetians in order to control the island and protect it from foreign invasions. Castello was renovated some 10 years ago. It has an octagonal lay-out and features openings for cannons, archways, a central tower, lofts and cannons. The excavations that preceded the restoration revealed a sewage system.