The Fortress of the Knights of the Order of Saint John dates back to the 14th century and is built on the summit of a small hill overlooking the harbor. Characteristically, the rock has a reddish color (Castello Rosso), from which the island also derived its name.
Palaiokastro is the oldest monument on the island. Inside the Castle, architectural elements of ancient buildings and cisterns are preserved, while excavations have brought to light Neolithic tools. Within the castle grounds, you will see the churches of Panagia of the Castle, Agia Marina, Agia Paraskevi, and Agios Georgios.
The Cathedral of Saints Constantine and Helen, dating from 1835, dominates over Mandraki. It is a church of exceptional artistry, in the architectural style of a three-aisled vaulted basilica, featuring rich iconography, a marble iconostasis, and an impressive marble bell tower. The roof of this cathedral is supported by twelve monolithic granite columns transferred from the Temple of Apollo in Patara of Lycia, Asia Minor.
The Archaeological Museum of the island has a remarkable collection of exhibits dating from antiquity, the Byzantine era, and modern times. Additionally, the museum hosts folk art exhibits that showcase the cultural heritage and traditions of the island.
The "Blue Cave," also known as the "Cave of Parastas," is located on the southeastern coast of the island. It is one of the most beautiful sea caves in the Mediterranean and a very rare geological phenomenon. The cave is also known by the name Fokiali due to the seals that inhabit it. Its interior is adorned with wonderful stalactites and stalagmites.