The Isa Bey Mosque located on the slope containing Ayasoluk Castle and the Church of St. John. This is the oldest active mosque in Turkey and it is considered one of the most beautiful examples of Anatolian mosque architecture. It was built in 1375 by the order of Isa Bey of the Aydinogulları Emirate. The construction used stone pieces and columns brought from ruins at Ephesus and the Temple of Artemis. The mosque stood in ruins for many years but was restored in 1970 and reopened for prayer. A second series of restorations were done on the structure in 2005.
The Isa Bey Mosque is built 51 meters by 57 metres. This asymmetrical design different from the typical construction of mosques. It once had two minarets, but the right one was destroyed in an earthquake. A monumental entrance on the western gate was surrounded by an elaborately carved moulding. Through the door was a colonnaded courtyard with 12 columns that made up the porticoes. A fountain in the center of the courtyard would have offered an area for worshipers to perform their ablution before praying.
The entrance to the Mosque itself was through a triple vaulted doorway. The door which faces west is ornately decorated with a dedicatory inscription. The mosque was covered with two central domes decorated with elements of Selcuk architecture. These domes had pendentives and stalactites in the window moldings, mosaics, and painted tiles.
The western facade is decorated in the style of Konya Seljuk. The upper parts of the windows on the left are decorated with inscriptions about the life of the Prophet. The right facade are decorated much differently. This is one of the first mosques to contain two congregation places and it shows the transition from Seljuk to Ottoman art.