A Roman marching camp at Pen Plaenau, measuring 17.4 hectares (43 acres), was discovered from the air in 2003 by Toby Driver of RCAHMW and by Hugh Toller. It lies on the eastern slope of the Berwyn mountains six kilometres north-west of Llanarmon Dyffryn-Ceiriog in an area where there is no evidence of Roman military activity, or occupation, apart from a third-century coin hoard from Sarffle five kilometres to the south-east.

An initial survey defined the extent of the camp and identified three internal clavicula-gates. In May 2004 a detailed survey of the camp was undertaken by Hugh Toller. The camp was intended to be rectangular but the west rampart bends at the west gateway, and has distorted the north-west angle so that the north and south ramparts are not parallel. These irregularities can be explained by the difficulty of laying out straight alignments without direct lines of sight. The straight south rampart was probably the base line for laying out the camp, and the west and east sides were intended to be laid out at right angles to it. A slight error of two degrees westwards occurred in both these angles. The rampart is well preserved for most of the circuit apart from the north-west angle and most of the north side where it is obscured by peat. Fortunately, enough of the west and north ramparts survive for the position of the north-west angle to be fixed with reasonable certainty. The ditch survives up to one metre deep everywhere except on the south where it has been obscured by the slope. Three entrances have been identified and a fourth has probably been lost under the peat. Part of the south and east sides have been used for fence lines since the late nineteenth century. There are three surviving clavicula-gates with curved internal rampart. They have ditch causeways and are approximately ten metres wide as are the radii of the claviculae. This probably equates to 30 Roman feet. At the east gateway the ditch appears to continue across the entrance gap but this is only because of later water erosion.

Information from Hugh Toller, 2006.

Published in: Toller in Archaeologia Cambrensis 153 for 2004 (2006), 13-21

T. Driver, RCAHMW, 20 June 2008