In my quest to visit all 120 CADW sites this was one of the first I visited back in 2007. Difficult to find and an O/S map is vital. Despite being a CADW site there are no sign posts to help. Park as best you can at the entrance to the farm (not much room and the tractors were fast and frequent when I visited). Take the 'path' through the trees oposite the farm entrance and make your way along the right hand side field until you come to a gate. Climb over the gate, through more trees and you come to the hillfort. When I visited in the summer the grass was high and it wasn't easy to spot the defences although they could be made out. Perhaps best visited during the winter months when vegetation is low? - and less tractors about!
Walked up from Llanvair-Discoed (22.5.09). It took a bit of finding - the entrance that is, the hill itself is pretty obvious!
Walking along the road leading east from the village, you come to a junction at the picturesque Old Mill, taking the right turn, you start to climb up the hill - at this point I passed an overgrown path going off on the right hand side, but ignored this. Coming to the entrance to Coombe Farm, there is a path going off into the trees on the right. Obscured by vegetation on the left here is a small outlying earthwork. You then come to a gate into a ploughed and cropped field - the OS map shows a single black pecked line (not a right of way), which leads across the field into the wood itself. This had obviously been used by others, as there was a clear line across the field through the crops. I took this "path", which led into the wood. From this point there are nicely maintained gates and stiles, but there's certainly nothing to indicate that the fort is in state guardianship - it's certainly a contrast to the Cotswold hillforts, which are mostly either golf courses or a least low grass.
I entered the fort itself from the NE, at a point where a series of parallel earthworks running NW-SE hits the main circular enclosure. It's not clear what purpose these parallel banks and ditches served.
The banks are well preserved, although the ditches are choked with vegetation at this time of year. The interior of the main encloure is knee-deep in vegetation, but a great spot as it was totally deserted - I can't remember the last time I visited a state maintained hillfort that showed no signs of anyone visiting at all. The slopes on all sides are wooded, so there is not much of a view, but the sense of peace and quiet was terrific. I spent about 15 minutes sitting in the grass and then went off down to the SW. Here, a steep path scrambles back down in the direction of the road, eventually turning into the overgrown path I had ignored earlier. I still don't know what is the "official" way in, but it's well worth a visit!
From here I walked down to Caerwent, with its impressive Roman walls and rather excellent pub.
Llanmelin fort is run by CADW, so you can visit it (as long as you leave your dog at home). It was excavated in the 1930s, when finds confirmed its Iron Age roots, though it had been used later in medieval times. It may have been the capital of the Silures, before the wretched Romans whupped their behinds and built Caerwent to keep an eye on them, just to the south. The top of the fort has been cleared of trees, as you can see in this photo at 'Gathering the Jewels': http://www.tlysau.org.uk/en/item1/1248
There is also a little outlying defended area to the north-east, by the road at ST 463928.