Kastro-Kyllini is situated in the westernmost part of Elis and the Pelloponnese 12 kilometres west of Andravida and 40 kilometres northwest of Pyrgos. It covers the northern part of the Kyllini peninsula, which juts into the Ionian Sea. The area is quite hilly where forests and olive groves can be found, but to the east of the peninsula it is very flat. Farming is the main occupation on the flatter lands.
The Crusader fortress of Chlemoutsi is situated on the highest point of the peninsula, about 220 metres above sea level. Between the northernmost point Cape Kyllini and the town Kyllini the ruins of the medieval town of Glarentza have been excavated.
There are several popular beaches on the coast. The thermal springs of Kyllini are the largest beach facility in the western Peloponnese. The Monastery of Blachernae is southeast of Kato Panagia. The small island Kafkalida near Cape Kyllini, is uninhabited but houses a lighthouse.