The Garrison Tower was a military masterpiece. Built in the French style by William, Earl of Pembroke, it formed part of the curtain wall and dominated the Usk valley. Round walls enabled defenders to fire in all directions, making the tower less vulnerable to damage from attackers.

It was constructed around 1209 at a time of increasing tension both in the England of King John and the Wales of Llewellyn the Great. William's wife Isabella de Clare, who occupied the 'chief place in a council of the Marshall's men', lived at Usk, and her 10 children would have been familiar with it.