Gyarah Sidi is a modest but intriguing set of stone steps near Mehtab Bagh on the Yamuna's north bank. It is often described as an early Mughal astronomical observation point, though the surviving structure is fragmentary and easy to miss amid Agra's larger monuments.
Its value is as a short add-on for travellers already crossing the river for Mehtab Bagh and the Taj Mahal views. Do not expect a polished site; the pleasure is in finding a small technical relic of the Mughal landscape away from the main ticketed circuit.