The Fountain of Ahmed III stands in the square between Hagia Sophia and the main gate of Topkapi Palace, so you pass it on the way to both. It dates from 1728.
It is an outdoor monument: no ticket, no opening hours, nothing to queue for. Four facades each carry a fountain niche, the corners hold small sherbet kiosks, and the roofline has the wide painted eaves typical of the Tulip Period. Calligraphic verses run around the top.
Worth two or three minutes on the way past, and best photographed early or late when the square is emptier and the light is on the painted panels rather than above them.
From Hotel Yasmak Sultan the walk is about nine minutes.