Nisyros is one of the youngest large volcanic centres in Greece included in the active volcanoes along with Methana, Milos and Santorini. The oldest rock formations are 160 000 years old, while the youngest (prehistoric) are 15 000 years old and cover the entire island. Most of Nisyros is mountainous, volcanic and very fertile. The island is composed of volcanic rock formations, with swelled elevations that were formed by previous volcanic activities. The various rock formations, (basalt etc.) have been used for the construction of various buildings such as the
Paleokastro is a classical period acropolis of the ancient city. It is still in very good condition because of its high quality construction and represents one of the best fortification works of ancientry, basaltic andesite, one of the hardest rocks in the world. The main gate is composed by well-hewn stone blocks with a perfect fit of the connecting surfaces. The total length of the fortification is very big, as it surrounds the entire land-lubber site of the rocky hill above the south end of Mandraki.
The monastery of Panagia Spiliani is built at the top of a hill northwest of Mandraki. Around the monastery retained sections of the Paleokastro wall still exist, whereas the castle is protected by the «Castle of the Knights of Rhodes» built by the Knights of St. John in the early of 14th century. The biggest part of the monastery is built in a cave. The cave is divided into two churches. The north is dedicated to the Kimisi tis Theotokou (Assumption of Mary) and the south, Agios Charalambos (Saint Charalampos).
The Castle of the Knights of St. John is in Mandraki, built high on a rock at an altimeter of 150 meters. The castle is a reminder of the Knights of St. John from the island of the Knights of St. John passing from the island, who in 1315 created here also a magnificent castle that stands today monumental and mansion. Inside the castle is the Monastery of Panagia Spiliani dedicated to Kimisi of Theotokou (assumption) and is the patron saint of the island. The monastery is celebrating on August 15 and is great festival.