The oldest monuments of Nisyros are dated in the Neolithic times and ever since the Classic antiquity the acropolis called Palaiokastro (6th-4th century) has been preserved. The sturdy wall is made of black trachyte which is impressive for its size. Its thickness was 3-5 meters, every 25 meters it had square towers with a width of almost 10 meters, while some of its stones reach a weight of 3.5 tons. The western and northern part of the hill ends up in a steep and rocky shore which is the natural fortification. Info Part of the defensive system with towers and observatories that run through Nisyros and the surrounding islets were used to warn through smoke or fire about forthcoming dangers.