This is the Maiden's Tower. It's been located on the Bosphorus since 1110. According to the legend, a Byzantine emperor had the tower built for his daughter. An oracle had predicted her death on her 18th birthday and he thought that if she lived in the middle of the Bosphorus, she would escape this fate. Obviously, she did not, because as far as legends go, when an oracle predicts your death, you're doomed.