Taking the first left after the castle and driving up the very very steep windy road. I was delighted and surprised to find that my camper van was able to climb it. You will come to a crossroads with a phone box and post box.
Turn left here. Follow the road along you will see a standing stone on the right hand side.
Just after this is a small layby / bin store.
Take the next right. You have to go through two lots of farm track gates. Luckily I had my son with me, because there are lots of these around here.
A little while after the 2nd lot of gates you will see a public footpath signposted on both sides of the track. There is a stream that has been culverted under the track. Lots of room to park here.
Taking the path on the left, follow it around the side of the little hill and keep going left. I passed another circular structure that looked to me like a cairn. (I've added a pic) Pass through the gap in the old stone wall and then follow the track up hill.
When the ground levels the circle is on your right.
High above Harlech, you need to take the unclassified Road from Llanfair and park up on the left past the farm entrance. Hop over the fence and walk up the bracken filled steep hill. The Circle will appear on your left as you go over the hill in a clearing. There are signs of a possible 2nd circle 50 yards away.