The castle is near the picturesque village of Thermisia east of Ermione and very close to a very nice lagoon. The castle was built around the end of the 12th century.
The castle takes its name from the ancient temple of Demeter Thermasia that was nearby, at the lagoon, not on the rock. [Many thanks to John Haanity for the drone video].
About 15 minutes drive from Ermioni and a bit of walking but the views from the castle are excellent. You will first meet the laggon on the right hand side just before the Thermisia village.