The Kalymnos Castle is built on the top of a steep cliff and was built in the 10th century, during the period of the Macedonian Dynasty. Its walls reach 6-7 meters high while it is fortified from all sides. The castle has two entrances: the main one, on the southwest side and a smaller one, the "portelli", in the northwest. The houses inside the castle were built according to Byzantine standards, next to each other and separated by narrow streets. The castle was able to accommodate 2000 people.
Today there are ten churches, dating back to the 14th - early 19th century, two water reservoirs, the dilapidated houses and the "Gourna" that served as an oil mill. In the middle of the eastern wall there is a tower, on which cannons are in place.