The Roman fort at Tomen-y-Mur is described in detail in Frances Lynchís guide to the ancient monuments of Gwynedd. The complexity of its history makes it fascinating but difficult to interpret, and you should allow at least an hour to do it justice.
If you approach the fort from the south along the line of Sarn Helen, you will have to jump across a little stream. The remains of the Roman bridge abutments should be just downstreamof the crossing. To your left, in the angle of the track and the stream, were the fortís guest house, the baths and a small civilian settlement (probably little more than a couple of squalid alehouses and a handful of women of negotiable affections). The large flattened area ahead and to the right of the track was the parade ground.
The fort itself is on private ground but access has recently been negotiated and there are stiles and gates from the track. Further along the track, just before it meets the metalled road, is the amphitheatre. This is much smaller than the amphitheatres of Caerleon and Carmarthen and some archaeologists have suggested that was used as a training area rather than a place of entertainment.
This must have been one of the bleakest postings in Roman Britain, but the site was reoccupied in the medieval period. William Rufus was there on campaign in 1095 and may have ordered the building of the motte which gives the fort its modern name. Even earlier, a tradition in the Welsh legends of the Mabinogion names it as Mur-y-Castell, the palace of the rulers of Ardudwy. It was at Mur-y-Castell that Lleu Llaw Gyffes lived with his beautiful wife Blodeuedd, the lady made out of flowers. The whole landscape of this area is saturated with traditions of these strange legends.