Alice de Lacy died childless in 1348, allowing Edward III to grant Builth to the Black Prince. In 1359 the prince restored a rental arrangement with the still-powerful Mortimer family but kept the right to garrison the castle in wartime. That retained military authority mattered when Owain Glyndwr's forces attacked in the early fifteenth century. Sir John Oldcastle, described here as an apparent inspiration for Shakespeare's Falstaff, commanded the garrison. Repairs costing 400 pounds in 1409 show the crown still spending enough to keep the site serviceable after the crisis.
Chapter · 7:48-8:47
The Black Prince's garrison right
The Black Prince rents Builth back to the Mortimers but retains wartime garrison rights later exercised during Owain Glyndwr's revolt.