The Thornbury Castle has been described as the last genuine castle, or rather private house with defensive features, ever raised in England. This is probably true if we ignore Scottish border territory. It is testimony to the ambition of Edward Stafford, Duke of Buckingham, who began building here in 1511. Ten years later, Henry VIII had him executed on a charge of treason. It was alleged that the duke had raised a private army in the Welsh Marches, in defiance of the Tudor laws against such practices, and Thornbury Castle may have been another factor weighing against him.
The castle follows the standard quadrangular layout of later medieval times, and is provided with an outer courtyard large enough to house a sizeable body of retainers. So here, as elsewhere, the hired levies were kept away from the duke and his personal household, though whether this arrangement reflects mistrust or the social hierarchy is a moot point.
Two long ranges of retainers' lodgings back onto the outer curtain. This curtain has three square flanking towers, the angle tower is set diagonally, several intermediate turrets and a liberal supply of gun ports and arrow slits. The main entrance, flanked by semi-octagonal turrets to front and rear, was furnished with a portcullis in traditional fashion. The south wall of the outer courtyard was never built and on the east lies the inner quadrangle.
Clearly, the west façade of the inner curtain was intended to look uncompromisingly defensive, with massive octagonal towers at each end and a twin-towered gatehouse in between. However, this front appears woefully squat because it was left in 1521 at less than half its intended height. The north range, with square flanking towers, is similarly truncated and the east range, which would have contained the hall, was never even begun.