This is the Halil Paşa tower. Its inner diameter is 23,30 metres, the inside one 10 metres, so we must conclude the walls are 6,5 metres or so thick. The external diameter higher up is 15,50 meters and the walls slim to a mere 2,8 metres. The height is 33 metres. It is not round, but 12-sided and has (had, looked pretty empty inside) six floors. I managed to get locked into the cellar (the guards did not check if someone had gone down that stairs when they locked a grate), but some shouting brought rescue. From the cellar I took several pictures looking up.