what: Agra Fort

The amount of carving and detail work is amazing. Even more so, this city/palace was full of symbols from all major religions. The Hall of Public Audiences Guest rooms (?) looking out along the banks of the Yamuna River.
I do not think I would want to be an army trying to attack the gate... Agra Fort began life in 1565 as a principally military structure. It is hard to appreciate how "thick" the outer walls are without being there.