Ilyas Bey Mosque, which is surrounded with a courtyard between the ruins of Miletus, was built in 1403 by Ilyas Bey (1402-1421) of the Turkish Mentse Emirate (Menteseogullari), who ruled in southwestern Anatolia for over a century until the Ottoman annexation of 1424. The building was repaired in 1905 for the first time when the dome was covered with brick tiles. It was restored from 1955 to 1972 by the General Directorate of Religious Endowments.
Ilyas Bey Mosque is a part of a complex, with the ruins of madrasa units on three sides of the mosque, and the tomb of Ilyas Bey to its north, all enclosed within a walled precinct. The mosque has a square plan, whose sides differ only half a meter, with an approximate length of eighteen and a half meters per sides. The portal on the north façade, which projects outwards, consists of a triple archway with stalactite capitals set in a large arched recess. Accessed by three steps the triple archway is framed by marble molding on the sides and topped by three blind arches without keystones, with polychrome tympana decoration. The tympanum of the central arch contains three lines of carved Arabic inscriptions that precise the patron and give the date of completion of the mosque, 806 AH. While the wooden screens of the two side arches have remained, the wooden door of the central arch has not survived.
Inside, the prayer hall is surmounted by a semi-spherical dome fourteen meters in diameter. The dome, which is made of brick and covered with tiles, sits on an octagonal base that rests on the four walls. The most important element of the interior space is the highly ornamented marble mihrab, with an imposing frame about seven and a half meters high and five meters wide. Muqarnas carvings decorate the base and crown of the mihrab niche and border the mihrab frame, which also includes two panels with lantern motifs, carved Quranic inscriptions and geometric designs.
An arched door on the northwest corner of the prayer hall leads to the base of the brick cylindrical minaret, which collapsed in the 1955 earthquake. Only a section o its base remains on the corner of the roof. The east, west and south facades have a pair of lower and upper windows; all are ornamented in different ways. The first row of windows of the east façade has Quranic inscriptions, while the upper row is decorated with geometric motifs and stalactites. Mosaic tiles and colorful marbles with palmet motifs were also used in the decoration of window frames. The windows on the qibla wall are slightly larger than the others. The two-meter thick walls of the mosque are constructed with stone blocks found in ruins of Miletus and paneled with high quality marble. This marble cladding has helped preserve the structure for over five hundred years.
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