The natural division of the Kastelli hill into two different levels was wonderfully exploited by the Kalymnians, who built a Byzantine fortress in this beautiful area in order to control the sea passage between Kalymnos and Telendos.
The strategically positioned fortress was being in use until the 10th century AD, and until then 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.