Large Stepwell in Kumbhalgarh Citadel, near Udaipur (India)
Stepwells, also called bawdi or baoli, are wells or ponds in which the water can be reached by descending a set of steps.
This is probably one of the most interesting part of what's left of the fortress, but it is hard to find and not on the "tourist circuit". Requires some scrambling, since the Step-well was protected by its own fortified wall.
Kumbalgarh is a fortress situated 82 km North-West of Udaipur. See this en.wikipedia.org/wiki/Kumbhalgarh.