The Alaeddin Mosque is the oldest mosque in Eskişehir, built in 1267 under the Seljuk sultan Kayqubad I — earlier than the Ottoman dynasty's rise. Inside, the flat wooden roof is held up by twelve carved wooden columns; the technique, common in surviving Seljuk timber-roof mosques across central Anatolia (Beyşehir, Afyon, Ankara), is rare and very different from the domed Ottoman pattern that came later.
The mosque has been repaired and re-roofed many times. It is still in active use; visit outside prayer times, leave your shoes at the entrance, and look up at the carved capitals and beams. A small explanatory panel near the door covers the structure's history.