It covers the northern part of the Kyllini peninsula, which juts into the Ionian Sea.

The Crusader fortress of Chlemoutsi is situated on the highest point of the peninsula, about 220 metres (722 feet) above sea level.

The municipal unit Kastro-Kyllini is subdivided into the following communities (constituent villages in brackets): The community of Kato Panagia (Greek: Κάτω Παναγιά) consists of the villages Kato Panagia, Ikaros and Moni Vlachernon.

The village Kato Panagia is directly adjacent to Kyllini, on the road to Lechaina.

Refugees from Asia Minor settled in Kato Panagia as a result of the Greco-Turkish War (1919–1922).

The community of Kastro (Greek: Κάστρο) consists of the town Kastro and the small villages Analipsi, Kalamia, Karavaki, Loutra Kyllinis and Psili Rachi.

The fortress was built by prince Geoffrey I Villehardouin of the Principality of Achaea in 1220-1223, who called it Clairmont (from which Chlemoutsi is a derivation ).

The most famous baths also offers inhalation therapy, said to assist in asthma and skin problems.