I visited this site a few years ago – this is what I can remember:
Directions – Head north towards Penmark off the B4265 / A4226 roundabout near Cardiff Airport. Before reaching the village take the turning on the right and head towards Llancarfan. Before you reach Llancarfan take the turning on the right signposted Moulton. This road runs along the southern defences of the Hillfort.
The lane is narrow and due to a hedge there was no obvious access to the site. In the end I parked at the farm and knocked on the door to ask for permission. There was no answer although it looked as though there was someone in. I took the opportunity for a quick look.
Access is easy from the farm (I seem to remember they had their own little path up to the site?) The inner Hillfort is fenced off and there were sheep grazing when I visited. There didn't appear to be much left of the defences although the middle of the site was pretty flat. I was conscious of being 'spotted' so I didn't climb over the fence and didn't stay too long.
I have since discovered that there is a public footpath which runs along the northern edge of the site but not into the Hillfort itself. Perhaps the defences are better preserved on the northern side?