Dating from the 4th century BC, the ancient walls above Nisyros, accessed from the path behind the monastery and medieval castle, are one of the Aegean's best surviving fortifications from the Classical period. The massive Cyclopic stones are remarkable, as is the one standing gateway. You can climb the steps and stand on the wall itself (be aware: no barriers), a superb sunset lookout.
It is believed to have been part of a defence strategy of Mausolus of Caria who, for a brief time from 355 BC, had control over Rhodes and Kos, among other smaller islands.