You can park in Trefil by a small play park, there is actually plenty of parking here and it is normally very quiet. You can follow the road up to a gate, which you can open and you are then on a vast expanse of common land. There are no paths that I could discern to Twyn Ceilog, so I just followed sheep trails and slogged through boggy areas and overgrowth until I found the summit.
The summit is marked by remains of an old wall / partial cairn or just pile of rocks. This was more than I was expecting, so it was good on that front. Probably not a place I will ever return to, but was not the worst walk.
I added a rock on top of the pile. I wonder how long it will stay there.