The castle's early life was royal. The museum handbook notes that its newly restored spaces reflect 'the types of furniture, textiles and painted decoration that could have greeted Henry I when he stayed in Norwich in 1121.' The keep was one of the largest and most sophisticated Norman structures in England, a statement of royal power in a city that was, at the time, one of the most important in the kingdom.

But royal castles have a way of outliving the purposes they were built for. By the medieval period Norwich Castle had accumulated other uses, among them serving as the county gaol. Generations of prisoners were held here, many never returning home. The castle's battlements became a site of public execution. On 7 December 1549, Robert Kett — leader of the peasant revolt now known as Kett's Rebellion, convicted of high treason under Edward VI — was drawn from the Guildhall to the castle and hanged in chains from a gibbet on the west face. His brother William was executed at Wymondham the same day. A roundel by sculptor James Woodford on the bronze entrance door of Norwich City Hall still depicts the hanging.

The gaol continued to function into the Victorian era, and the castle's grounds became a stage for some of the most public moments in Norfolk's criminal history. On 21 April 1849, James Bloomfield Rush — convicted of the double murder of Isaac Jermy and his son at Stanfield Hall near Wymondham — was marched to the scaffold at noon and executed by hangman William Calcraft in front of thousands of spectators. Rush had defended himself at trial, prolonging the proceedings and keeping the case in the national press for months. He was buried in the castle grounds. A life-size waxwork of him was displayed at Madame Tussauds in London from 1849 until 1971.

The city of Norwich eventually bought the castle to use as a museum. The conversion, undertaken by architect Edward Boardman, swept away the gaol's cell complex within the keep, installed flooring and balconies, and built Norman-style arches to support a new glazed roof. The exercise yards and the gaoler's house made way for gardens. For over a century the castle served Norwich as its principal museum and art gallery.

Not all of that time was quiet. In February 2012, four men forced open a display case and attempted to steal a rhinoceros head, intending to sell its horns. Museum staff tackled them and stopped the theft. Five days later, artefacts belonging to Admiral Lord Nelson — valued at £36,800 and including a gold mourning ring worth £25,000 — were stolen. The horns were replaced with replicas; some of the Nelson pieces were eventually recovered.

The Royal Palace Reborn project began in 2020, was delayed by the pandemic and by discoveries made during construction, and was completed in 2025. The keep's floors were reinstated in their original positions with rooms recreated — Great Chamber, banquet hall, communal toilets — and furnished and decorated. A new lift provides wheelchair access to all five floors, including the battlements. The castle that Henry I visited in 1121 has been made legible again.