Dolforwyn Castle was built between 1273-77 by Llywelyn ap Gruffudd (Llywelyn the Last) as a forward position in his territory, and overlooking the English lordship of Montgomery. This rectangular castle crowns a ridge along the Severn valley, and was obviously designed to act as a sentinel over Llywelyn's south-eastern frontier. Its initial construction led King Edward I to write to Prince Llywelyn in 1273, forbidding him to build the castle. The prince replied, with a masterpiece of ironic politeness, that he did not require the king's permission to raise a stronghold in his own principality. Dolforwyn was, however, taken by Roger Mortimer after a fortnight's siege in 1277, and given to the Mortimers, a powerful marcher family. The castle was kept in repair for some years, but was ruinous by 1398.

Source:
http://www.castlewales.com/dolforn.html