 Dolbadarn Castle was built in either the 1220s or the 1230s by Llywelyn the Great , at the base of the Llanberis Pass , overlooking the lake of Llyn Padarn in North Wales . Traditionally the Welsh princes had not constructed castles , instead using undefended palaces called <unk> , or courts . From the late 11th century onwards , the Normans had advanced into Wales , taking lands in the north and establishing a band of occupied territory in the south called the Welsh Marches . During the 12th century some timber and earthwork castles began to be built , but in small numbers . 
