Park in the castle car park. To gain access to the cache you don't have to pay admission to the castle as a footpath goes around the base, if approached by the castle staff explain you are walking the footpath and not going to the castle.
Carreg Cennen Castle has absolutely everything you could imagine to expect to find in a romantic ruined castle. Perched on the edge of a 100m cliff, the castle has spectacular views in every direction - indeed you'll see it from quite a distance as you approach! If you're going to visit the castle itself, plan to bring a good torch as the castle is also famous for having a natural limestone cave within its confines which can be freely explored - originally it is believed that this was its water supply. Some call this its dungeon! Torches can also be hired. Note that there is an admission fee for the grounds of the castle itself which is payable at the tea room at the bottom of the hill near the car park (or free if you are a member of Cadw or one of the organisations which they have reciprocal agreements with).
There are two routes to the cache. For the more interesting walk turn right just before you reach the Farm/Tea Room and follow the yellow arrowed posts (marked Short Route) around the base of the castle.The quicker way is to head up to the castle, when you reach the shed, go through the left gate (down the hill).
On adopting this cache in early 2010, I discovered that the original log book was still in place and is possibly one of the longest continually running log books for a cache in Wales. As I feel this forms a historic record of caching in South Wales (in a relatively popular area), I've restored and photographed it and made the log book available on-line for others to study and enjoy.