The Orhan Gazi Cami (early Ottoman, one dome, late 14th century) and the nearby türbe for Edebali (14th century too). A guide I used indicates it has a special meaning for Turks, without explaining why. I was informed by a viewer as follows:

"This mosque has a deep meaning for the local people. The mosque is known as "Kursunlu Cami" in that region which means "Mosque of Bullets". The original mosque was full of bullet marks left from the days of Greek invasion in 1921. Besides that, the ruins of other minarets in other pictures are from the same days. The previous city was even bigger and more developed than today's Bilecik. It was burnt down and devasted by the Greeks twice during our Independence war. Now the city could only dream and miss its glorious days. Seyh Edebali is the moral founder of the Ottoman Empire, his advices and guideness has established the principles of the new born state.He was the wise, respected holy man who first promised "an empire that will rise from a small clan" which will rule 3 continents."

I am slightly doubtful about the "kursunlu" part as in many other cities kurşunlu refers to a mosque having a roof made of lead (kurşun). For the rest I think the comment explains itself, thanks for the information.