Agra's Red Fort
Akbar laid the stones on top of an existing brick fort used by the Lodhi Sultans, who ruled northern India before the conquering moghuls swept down from Uzbekistan.
The fort has been in continuous use as a military defense since (at least) the 11th century A.D. Even today, about ¾ of the fort is occupied by forces of the Indian Army.
This is the Amar Singh gate.