Piskopi 'lookout' took its name from its mountains on which the Knights of St John built seven castles and watchtowers, most of them with commanding views over the surrounding seas. Piscopi or Tilos is 14.5 kilometers long from northeast to southwest and about 8 kilometers at its widest point. It is about halfway between Rhodes and Lango (Cos) and 15 kilometers southeast of Nisyro (Nisyros).
The most important fortified stronghold was the castle on the summit above the present day village of Megalo Chorio. In the Knight's period the villagers lived within the castle walls as protection from pirates. The hillside had been the site of an ancient settlement and parts of the fabric of the castle, in particular the lower courses, the gatehouse and the corner towers were largely constructed from reused Classical masonry.
From the castle are clear views across the sea to Nisyro. The garrison kept watch over the channel and lit a beacon as a signal to Rhodes and the other islands if any shipping was seen passing through the strait.