This is a cracking site to visit. Probably the best Motte and Baily site I have been to. As you near the castle there is a phone box and a large are next to it where you can park. The castle is in a field across the other side of the lane - easily seen from the phone box. Access is via an open gate. Just one word of warning - the ground approaching the castle is very boggy and wet - even though it doesn't look it!
There's an excellent example of a motte and bailey castle here, with a well preserved ring keep, but that's not at all relevant to this web site. The area where the castle is built is not widely recognised as having prehistoric providence, but having visited a number of times I inclined to think there's a strong possibility that it was built on an existing Iron Age settlement. I base this theory on the unusually large size of the bailey in proportion to the motte.
In the book Saints and Stones (ISBN 1-84323-124-7) Davies and Eastham draw similar conclusions, suggesting that the Guise (or Gwis) family built their castle on, "what was probably a rath or Iron Age settlement". I need to do some more reading and see who else shares this idea.