Goodrich Castle, situated in Herefordshire between Monmouth and Ross on Wye. It was named after the Saxon lord Godric, who started the castle's construction in the late 11th century. The castle is administered by English Heritage, check here for opening times.
The rest of the module was filmed at Stokesay Castle in Shropshire. Visit if you can, it's one of the loveliest places in England.
(We returned to Stokesay later in the series to tell the story of Laurence of Ludlow, who owned Stokesay in the late 13th century; see the module 'First Parliaments').