Fort Agra is a 94 acre semicircular plan, its chord lies paralled to the river and its walls are seventy feet high. Double raparts have massive circular bastions at intervals, with battleents, embrasures, machicolations and string courses. Now don't ask me what half of that means I just thought some would like to know those details that wiki was willing to share. Four gates were provided on its four sides, one Khizri gate opening on to the river.

A wooden drawbridge was used to cross the moat and reach the front gate from the mainland; inside, an inner gateway called Hathi Pol ("Elephant Gate") - guarded by two life-sized stone elephants with their riders - added another layer of security. The drawbridge, slight ascent, and 90-degree turn between the outer and inner gates make the entrance impregnable. During a siege, attackers would employ elephants to crush a fort's gates. Without a level, straight run-up to gather speed, however, something prevented by this layout, elephants are ineffective.

The Indian military still uses the northern portion of the Agra Fort, mainly the Parachute Brigade.
The site is very important in terms of architectural history. Abul Fazal recorded that five hundred buildings in the beautiful designs of Bengal and Gujarat were built in the fort. (Wiki)