Buellt is located in Builth Wells between the Rivers Irfon and Wye. It is a medieval fortress constructed by King Edward I and was the first of his Iron Ring of Castles built to control Wales.

Buellt derives from old Welsh, ‘bu’ and ‘gellt’, meaning effectively an ox pasture. It was then anglicized to Builth. The Welsh kingdom of Buellt is mentioned in Welsh annals, and the site of the current castle was a seat of kings long before the Normans came. The first motte and bailey castle was built here by Philip de Braose, in his attempt to control the region. Control of the castle went back and forth between the Welsh and the Normans until Edward conquered the area completely in 1277.

Construction of the current stone castle began in May of 1277 and continued until 1282. Upon the death of Llywelyn ap Gruffydd, the last Welsh Prince of Wales,, Edward turned his attention to the construction of castles in Gwynedd, and the work at Buellt was left unfinished. Although the castle continued to be used, it burned in the 16th century, after which the stone was raided for building by the local populace. Today, what is left of the castle is located in the middle of a housing development, and all that remains are the mound, ditches, and earthworks. 

Castell Buellt plays a role my Lion of Wales series. It is also the Norman castle closest to Cilmeri, where Llywelyn was killed after the Mortimers lured him into an ambush.