The castle was first built by a Norman baron on the cliff above the River Nidd. Documentary evidence dating from 1130 records works carried out by Henry I, suggesting an earlier fortification on the site. After Becket's murderers used it as their bolt-hole in the 1170s, the castle became a possession of consequence. William de Stuteville was appointed Governor in 1173. When he died, King John reserved Knaresborough and nearby Boroughbridge for the Crown - even when paying out a 10,000-mark fine to the de Stuteville heir, he kept these particular castles. In August 1304, Elizabeth of Rhuddlan - daughter of Edward I - travelled from Linlithgow Palace to Knaresborough Castle to give birth. Her son Humphrey was born in September, the labour aided, according to the records, by a holy relic of the girdle of the Virgin brought from Westminster Abbey for the occasion. The castle had become a place where royal blood entered the world.

Between 1307 and 1312, Edward I and his son Edward II spent £2,174 - a staggering sum - rebuilding Knaresborough, including the great five-sided keep whose ruined eastern walls still impress visitors today. Edward II had a particular fondness for the place. He gave it to Piers Gaveston, the favourite whose intimate friendship with the king scandalised the English nobility. When Gaveston was besieged at Scarborough Castle by Earls who hated him, Edward II retreated to Knaresborough - powerless to save his friend, who would shortly be executed without trial. Philippa of Hainault, Edward III's queen, took possession in 1331, and Knaresborough became a royal residence. By 1561, a detailed survey of the castle recorded a building that was still standing but increasingly used for prosaic purposes - estate auditors, law courts in the hall. The keep was no longer a fortress. It was an office.

Parliamentarian forces took the castle in 1644 during the English Civil War. The fighting did little damage. The real destruction came in 1648, when Parliament ordered the dismantling of all Royalist castles - a systematic slighting designed to ensure no king would ever again command these strongholds. Knaresborough's walls came down not in the heat of war but cold-bloodedly, by order. Townspeople carted the stone away. Today, many of Knaresborough's town-centre buildings are built of what locals call "castle stone" - the literal bones of the fortress recycled into shopfronts and houses. Only the great keep with its missing eastern half, fragments of curtain wall, and the gatehouse towers survived in any height. The castle's last function was the strangest: in 1789, historian Ely Hargrove wrote that the surviving castle building contained "only three rooms on a floor, and measures, in front, only fifty-four feet."

Knaresborough Castle has kept ravens since 2000, one of them gifted from the Tower of London. Among them is Mourdour, an African pied crow whose Yorkshire accent went viral in 2018 when a tourist filmed her saying "Y'alright love?" to passers-by. The video reached news broadcasters around the world. The castle's grounds are now public leisure space - a bowling green and putting green operate during summer, and the ruins host events like the annual FEVA Festival of Visual Arts and Entertainment. The 14th-century former courthouse in the grounds is now a museum, displaying furniture from the original Tudor Court alongside exhibits about castle and town. Property of the Duchy of Lancaster, administered by North Yorkshire Council, the ruins have settled into something gentler than the fortress they once were - though the geology beneath the castle still puts on a show, with mid-Carboniferous sandstones at road level and late Permian grits and limestones above, an unconformity visible in the cliff face.