Chlemoutsi is a castle in Elis in the northwestern Peloponnese. It was the primary fortification of the Principality of Achaia, constructed on a hill with a panoramic view westward to the Ionian Sea and controlling passage into the interior of the Peloponnese. It was built between 1220 and 1223 by Geoffrey I Villehardouin who used the wealth of the Moreot church for its construction, and was consequently excommunicated. Despite the castle's formidable size and position, it seems to have witnessed no memorable sieges or battles. It was frequently used as a prison, and the Greeks captured at the battle of Makryplage in 1264 were held there. Chlemoutsi remained in Frankish hands until 1429 when it was taken by Constantine XI Palaiologos, then despot of the Morea, and used by him as a base for his attack on Patras. The despot Thomas Palaiologos kept John Asen prisoner at Chlemoutsi. It was taken by the Turks in 1460.
The surviving fortress is almost entirely Frankish. It consists of a large polygonal circuit wall and, at the summit of the hill, a powerful keep—an irregular hexagon, with sides 60-90 m long—and an interior court. The walls are in fact enormous halls, over 7 m wide with two stories, the upper supported either on vaults or with wooden beams. Living quarters were on the upper stories. Elaborate arrangements brought water from the roofs to huge cisterns under the floors.