During the 7th century, the time of the Arabian raids, the coastal fortress Kastelli was built at the homonymous location, on the opposite island of Telendos. It was built on a naturally fortified area which allows the nominal control of the sea area between Kalymnos and the neighboring island of Telendos.
The natural division of the Kastelli hill into two different levels was wonderfully exploited by the Kalymnians and it provided the Kalymnians with the ability to react directly in potential threats and defend themselves against attacks that came from land or sea. The entrance to Kastelli was and is carried out through gates that exist in the outer wall, while the water supply was being ensured by water tanks.
The intense and extensive human activity in the area of the fortress is witnessed by the abundant pottery, existing on the surface of the ground. Possibly it was in use until the 9th or 10th century at the present moment however we are not sure until when precisely it was inhabited.
Source: www.kalymnos-isl.gr