At the southern base of Ayasuluk Hill, this imposing mosque was built in a post-Seljuk/pre-Ottoman transitional style, when Selçuk was capital of the Aydın Emirate. An inscription in Arabic above the main entrance states that it was built in 1375. It is open to visitors (except at prayer times).
The two domes are resting on marble columns recycled from a Roman building at Ephesus; don't miss the beautiful mosaic in the south dome. İsa Bey Camii's minaret, built on an octagonal base, was being renovated at the time of research. There's a huge bust of the patron, the Emir of Aydin, opposite the main entrance outside.