I’ve not visited Crug-yr-Hwch (aka Mountain Burial Chamber) yet, but I understand it’s incorporated into a field boundary (like Tre Wallter Llwyd) with collapsed uprights. It may have been a Portal Dolmen, similar to Pentre Ifan. I get the impression it’s not been excavated.
In his book Prehistoric Preseli Figgis suggests that the tomb’s position between two small rivers (Foel Drygarn and Crug yr Hwch) might be significant.