İlyas Bey Mosque was built in Miletus by the emir of Mentese in 1404, following the restoration of his principality’s independence by Timur. It is a rare and important example of an earlier architectural tradition, as it bears none of the hallmarks of later Ottoman mosque building. The conversion of  Hagia Sophia into a mosque following the fall of Constantinople in 1453 AD greatly influenced Ottoman architecture, introducing grandeur, spacious interiors and large domes and semi-domes into the architecture of the period. İlyas Bey Mosque predates the fall of Constantinople by 50 years, showing the smaller, less grandeur style of mosques that could be found in Anatolia.

İlyas Bey Mosque has square in plan with a single large dome. While its minaret is now missing, photographs and drawings from the early twentieth century show that it was situated on the western corner of the building with a decorative brick construction. The building is mostly plain on the outside except for a highly elaborate facade around the door on its northwest side. There are some extremely fine details in carved stone on the main facade of the mosque, lattice-work over its windows and around the mihrab, sharing the use of carved stone common in Muslim Anatolia. The most outstanding decorative feature of İlyas Bey Mosque is the extensive use of stucco. 

In the hammam next to the mosque plain plaster surfaces of the walls are decorated with impressed blocks of floral and geometric design, forming a dado at waist height; elsewhere it is formed into stalactite patterns above arches and in corners. İlyas Bey Mosque is remarkable for its almost total lack of ceramic tile decoration that was so common in Turkish architectural decoration. There are some traces of blue ceramic tile decoration on the underside of the arches on the mosque’s main facade, but these areas are extremely small, given the size of the building.