The first Roman fort at Caersws was on a meander of the Severn north-east of the present town, but it was moved to a position where it could command the confluence of the Cerist and the Carno with the Severn. The new site was liable to flooding, but probably no more so than the old one. Its position at the meeting-point of natural lines of communication has made it an important road and rail junction. It was the focal point of Roman roads running along the Severn valley and to the north and south.
More recently, it bears out the theory of one Welsh archaeologist that Roman forts are likely to be found near railway stations. Caersws station is actually within the defences of the fort, though the rampart can most clearly be seen a little to the north of the station where it crosses the A470. The fort attracted a small civilian settlement and both were still occupied after AD 400, probably because of the strategic importance of the roads.
The road to Caersws from the north runs parallel with the line of the Roman road. Just after Llwyn-y-gog Farm, in the fields to the right of the road, is the fortified settlement known as Gwynfynydd. This still has an impressive bank and ditch but it is not in a defensible position. It is probably no more than a fortified farmstead, and as it is so near to the Roman road it may be Roman in date (though not in inspiration).
More on the archaeology and landscape of the area on the Clwyd-Powys Archaeological Trust web site.