Usk Reservoir: How to get there
The reservoir is 6 miles from Trecastle and is signposted off the Llanddeusant road, which is a single track lane running from Trecastle.
Coming from the castle, you turn left at the Tafan y Garreg pub, take the mountain road for a few miles until you get to a T junction (after you have crossed over the mountain itself and over two cattle grids). Do not take any turnings off this road to the left or right until you get to the T Junction after some farms.
Turning right at this T junction would get you to Trecastle. Turning left will take you along the Llanddeusant Road and you will after a couple more miles come across a right hand turning off this road, signed to Usk Reservoir. The Reservoir is then a half mile or so down this lane.
When doing an anti-clockwise circular walk from Usk Reservoir Dam (where you park) you sometimes come out on to the Llanddeusant Road further along. It may seem a way to go on foot but you are actually quite close - a mile from your car at most. You have to turn left, and then take the lane first left back to the dam to Usk, this now being the lane signed to Usk Reservoir that you originally drove down.