Visited 24th April 2005: I found it tricky to park on the A4120, then a bit scary walking the boys along the road. There's not a lot of traffic, but nobody is really expecting pedestrians.
The fort itself is extremely small, only really big enough to contain one family. It's the hillfort equivalent of a farmstead, built on a small promontory above the Rheidol valley. There are one or two hut platforms visible within the defences, but they're too subtle to photograph.
The remaining defences are pretty meagre, and unfortunately the strategic position of the fort isn't as obvious as it might be because of the surrounding trees. It's still a beautiful place to visit on a fine day.
Although I wasn't aware of it during my visit, there is permissive public access to Pen-y-Felin Wynt (hats off to the land owner!). If you want to see the approved route, visit the Countryside Access in Wales web site.