They are the fortresses built on the narrowest part of the Istanbul Strait by the order of Yildirim Bayazed in 1391. I was built during the siege of Istanbul Strait. It was served as a safe passage from Anatolian Side to the European Side of the Strait.
The castle outside serves as a fortress. The east-west diameter is 65 m, north-south diameter 80 m, and the thickness of the ramparts is between 2 and 5 m.
There are braziers on the ramparts. There are three cylindrical towers that were built to protect the castle.