Having been almost completely redeveloped in the mid 1970s, the mainly industrial Bozüyük's only building of any importance is the Kasim Pasa Camii, erected by Sinan. About 20km/12.5mi east, on the Eskisehir border, lies an early Bronze Age settlement mound, Demircihüyük, finds from which can be seen in the museum in Eskisehir.
Situated on a hillside at a point where the Karasu, flowing down from Kandilli Dag, enters the Sakarya, this small town was known in antiquity as Leukai and more recently as Lefke (up until 1921 the population was still predominately Greek). South of the town is a narrow and very beautiful river gorge with rocky walls up to 100m/328ft high, contributing to some spectacular scenery. In the west part of Osmaneli are the well-preserved ruins of a large Byzantine church of indeterminate date (probably Neo-Byzantine) and, not far from it, a building which when in Greek hands was used as a silk-spinning mill, and also the owner's house.
About 25km/15.5mi southeast of Bilecik, on the old overland route from Eskisehir to Iznik, the district town of Sögüt occupies the site of ancient Thebasion. Prior to the Ottoman conquest Sögüt was the seat of the Oguz leader Ertogrul (d. 1289), father of Osman and therefore progenitor of the Ottoman dynasty. In the mid 13th century Ertogrul sided with the Seljuks against the Mongols, being rewarded by the Sultan Alaeddin Keykubad with the gift of a modest piece of feudal land in the area bordering Byzantium. From this base he fashioned the beginnings of what later grew to be the Ottoman Empire.Ertogrul's mausoleum, in the Old Ottoman style, endowed in the early 15th century by Mehmet I and altered several times, stands some 2km/1.25mi north of the town. The türbe is surrounded by other family graves, thirteen in all. Up until the reign of Abdul Hamid II, the spring festival of the formerly Shiite Oguz (held on March ninth) regularly brought huge numbers of the now Sunnite Karakeçil nomads flocking to Sögüt on pilgrimage. Latterly the Yürük festival took place in autumn. In the town itself stands the Ertogrul Gazi Mescidi endowed by Abdul Aziz (1861-76). This small domed mosque with a low minaret topped by a onion dome is said to stand on the site of an earlier mosque built by Ertogrul himself.