The Welsh rebellion of 1400, led by Owain Glyndwr, resulted in an edict from Henry lV to strengthen and supply castles in south Wales against the rebels. The castle was besieged in 1405 as the English forces sheltered within after fleeing from Grosmont castle. Surprised by the determination of the English soldiers, led by Lord Grey of Codnor, Glyndwr’s men were routed and fled to the marshy ground by the river, and many were taken prisoner. The massacre of 300 of these below the castle walls was a brutal reprisal indeed.

William ap Thomas held Usk after 1431, a supporter of the Yorkist cause. Both he and his son, William Herbert, are recorded as ruthless and highly unpopular. Herbert was created 18th Earl of Pembroke in 1468, and carried out improvements to make the castle more comfortable – the large windows and fireplaces in the Keep date from this time. William Herbert was killed at the Battle of Edgecote in 1469, and Usk Castle seems to have declined from then on, superseded by the large new castle at Raglan which had been built for the new weapons of warfare, cannons. By 1564 Roger Williams was accused of taking stone from the castle with which to build his new house, Ty Mawr, in Old Market Street.

By the time John Humphreys came to live in the gatehouse in 1908, the castle had the appearance of a farmyard overgrown with ivy and large trees, although the gatehouse had become a ‘gentrified’ town house. With the purchase of the castle in 1933 for £525, which included one donkey and one flag, the castle began to come back to life, as Rudge Humphreys spent the 1930’s excavating parts of it and planting the bulbs and shrubs which today visitors appreciate as creating a place of beauty and tranquillity.