Buellt Castle (Builth Wells for the English) was the seat from which the Mortimers lured Llywelyn ap Gruffydd to his death near Cilmeri on 11 December 1282.  It was a major Edwardian Castle of its time, but all of the stone work as disappeared.

“Builth is nothing more than a series of earthworks – nothing visible remains to give testimony to the structure which once stood at the site. By 1183, documents record a clash here between the Welsh and Normans, and much of what we see reflects this original motte and bailey fortification. During the next 90 years, the castle saw repeated conflict and changed hands between the Welsh and English on several occasions. By the 1240’s masonry structures were established at Builth; however, it was as the result of Edward I’s initial campaign against the Welsh in 1277 that Builth’s modest stronghold was refortified and transformed into a formidable fortress.

…The final product of Edward’s remodelling effort at Builth was a castle centered atop a motte which supported a great round keep (the traces of which are barely visible today) and was enclosed by a small “chemise”, a masonry wall defended by 6 towers. The two Norman baileys remained, encompassed by a curtain wall and accessed through a twin-towered gatehouse which may have been similar to the gatehouse that still guards Rhuddlan Castle. Other structures included a kitchen block and the great hall, a chapel, and residential quarters. Apparently, construction was stopped at Builth in 1282 although the work on the gatehouse may not have been complete.”  http://www.castlewales.com/builth.html