The module begins in Conwy Castle, and the helicopter sequence shows three other castles built in the decades after Edward I's conquest of Wales: Canaervon, Harlech, and Beaumaris. Instruments of oppression in their day, they're impressive monuments to the glory days of English castle-building.
The ruined tower where we filmed the Llewelyn ap Gruffydd sequence is Dolbadarn Castle on the Llanberis pass through Snowdonia, immortalised in a famous painting by J.M.W. Turner. Dolbardarn was built by Llewelyn's grandfather, Llewelyn the Great.