Most of this fort was obilterated when the Wallace Monument was built in the 19th century. The Abbey Craig is an old lava flow rising out of the flat, low Carse of Forth, and it's defensive properties are clear to see. There is a sheer drop on the W side, which affords views along Blairdrummond Moss towards Loch Lomond, while on the E side, which looks along the Carse of Forth, had a timber-laced heavy stone wall, enclosing an area of 175 ft by 125 ft. Look hard in the undergrowth around the Monument and you can still see vitrified material.
You can walk the grounds freely to look for the remains of the fort, but you have to pay to get into the Monument itself. I would highly recommend this. Even if you're not really interested in the history of William Wallace or seeing his enormous sword, it's well worth paying just to go straight up the narrow stairs to the top, where you get amazing views along the Ochils and the Carse of Forth to the E, and along Blairdrummond Moss to the W. You can also get a great view of Dumyat from a unique perspective, and can look down on the nearby standing stone at Airthrey.
The Abbey Craig is very easy to find, as you can see the Wallace Monument from miles around. If you can find your way onto the A9 in the centre of Stirling, heading N on the dual carriageway section, then at the roundabout next to the bridge over the Forth, turn right. Head along past all the B&Bs until you come to another roundabout, which you want to go straight over and up the small B998. The road doubles back on itself as it climbs the hill and you will soon find yourself at the car park for the Wallace Monument.
Watch out for the hideous statue of Mel Gibson (no really!) dressed in his Braveheart garb.
From the car park, just follow the path up to the Monument.