Palaiokastro is the fortress of the ancient city of Nisyros and one of the best preserved in the Aegean area. The ancient city was built at least since the 8th century BC, above the modern settlement of Mandraki. The wall, as it survives today, dates back to the 4th century BC. Built on the brow and the slopes of the hill it led to the cliff from its west side, whilst from its north, it reached up to the rock where the medieval castle and the monastery of Panagia Spiliani lie today.

The total length of the fort is very large. The wall and the cliff mark an area of approximately 130.000 m2, with total perimeter of almost 2 km, while the weight of some of the stones that make up the wall reaches 3.5 tons. The construction of the costly and strong fortifications could be attributed to the effort of the tyrant of Caria Mausolus and his successors, who for a short time (355-332 BC) had acquired control of Rhodes, Kos, and the neighbouring islands, to create a powerful defensive front in the region. The local volcanic black stone (basaltic andesite) was used for its construction. Palaiokastro is located in the village of Mandraki and the admission is free.