Palaio Pyli, Medieval castle ruins in Kos, Greece.

Palaio Pyli is a ruined hilltop castle on the island of Kos, sitting on a rocky ridge with stone walls, seven towers, and the remains of several churches. The site overlooks a green valley on one side and opens toward the Aegean Sea on the other.

The castle was probably built in the 11th century. In the 15th century the Peleten moved into the fortress and founded the village below it, hidden from the sea and thus protected from pirate raids. In 1830 a cholera epidemic led to the abandonment of castle and village, and most inhabitants moved to Pyli, also called New Pyli.

The village church, Ypapanti, still stands and holds 14th-century frescoes; the other two, Asomati Taxiarches Gavriil ke Michail and Panagia ton Kastrianon, were built in the 11th century and now stand largely ruined among the collapsed houses. The name Palio Pyli distinguishes the abandoned settlement from Pyli, the village below to which most inhabitants moved after 1830.

The path to the castle starts from the modern village of Pyli and runs through a forested hillside, so sturdy shoes are recommended. A small cafe near the top offers a chance to stop and rest before or after exploring the ruins.

The main gate of the castle features four semicircular arches dating from the 11th century, still standing after nearly a thousand years. Beneath the fortress walls, an underground cistern was carved out to collect and store water for all the people living inside.