Çeşme's majestic Genoese-built fortress dates from 1508 and was later repaired by order of Sultan Beyazıt II, son of Sultan Mehmet the Conqueror, in order to defend the coast from attack by pirates. Impressively restored, it now houses the town's museum. Rooms in the Umur Bey tower and around the bailey (inner enclosure) house archaeological and historical exhibits, and the terraced bailey is filled with historic gravestones and stelae. The battlements offer excellent views of the town and harbour.

The archaeological finds come from nearby Erythrae (now Ildırı), as well as from the Bağlararası Bronze Age harbour settlement (tools, a curious stone phallus) and the Roman era (superb gold diadems). Two rooms in the tower house a comprehensive exhibit about Russian Empress Catherine the Great and the Russian-Ottoman naval battle of 5 July 1770, which occurred in Çeşme Bay. A decisive engagement of the Russo-Turkish War of 1768–74, the Russians were triumphant and the Ottoman fleet wiped out.