The İsa Bey Mosque was built in 1374–75 for the Aydinid ruler İsa Bey and is one of the finest examples of architecture from the Anatolian beyliks — the small Turkish principalities that took over western Anatolia from the Byzantines before being absorbed by the Ottomans. The architect, Ali ibn Dimishqi of Damascus, drew on both Seljuk and Syrian-Mamluk traditions, and the building feels distinctly different from later Ottoman mosques.
The columns of the courtyard arcade are reused — pillaged from Ephesus and the Temple of Artemis a few hundred metres away, so that capitals of different orders sit on shafts of different stone, all of it ancient. The mihrab and the carved stalactite niches inside are exceptional. The mosque is active; you can enter outside prayer times, take shoes off, and women should cover their hair.