Henry VIII spent the late 1530s nervous about the north. The Pilgrimage of Grace had shown how shaky royal control could be, and Carlisle's medieval defences were antique. He hired von Haschenperg - a continental specialist in the new science of fortification against gunpowder - to modernise the castle and replace the old Botcher's Gate with a forward artillery work. The result was the Citadel: a Tudor fortress with two massive cannon towers astride the southern approach to the city. It was finished in 1541. von Haschenperg was dismissed in 1543, accused of having burned through huge sums to no purpose, though the structures he had begun were carried on by his successors. The thick walls and rounded forms were calibrated to absorb cannon shot rather than scale walls, the shape of warfare changing in the gunner's direction. From the south, you would have approached Carlisle through a stone mouth ringed with guns.

The Citadel ceased to be a serious fortification surprisingly fast. The personal union of the crowns under James VI and I in 1603 took most of the urgency out of the Anglo-Scottish frontier, and by the eighteenth century the towers were finding new civic uses. The complex was rebuilt in the early nineteenth century to designs influenced by Thomas Telford, with the eastern tower incorporating part of the original sixteenth-century structure. The Nisi Prius Courthouse and the former Crown Court occupied the two towers - both Grade I listed buildings today. For roughly a century and a half the Citadel was where Cumbrians stood trial, where the rituals of assize courts and quarter sessions unfolded in panelled chambers beneath ceilings the Tudors would have used to store powder. The courts moved out in 1992 when the new Carlisle Courts of Justice opened on Earl Street.

After the courts left, Cumbria County Council kept the buildings going as offices. The council had inherited them through the 1974 local government reorganisation that combined Cumberland with Westmorland into the modern county; the Citadel sat alongside a scatter of Victorian houses on Portland Square, Brunswick Street and Alfred Street North, and Lonsdale House in Lower Gaol Yard, all pressed into use as council workspace. In December 2016, as a cost-saving measure, the County Council consolidated everything into a single facility - Cumbria House on Botchergate - and the Citadel went quiet. In April of that year the council had already submitted proposals to government for funding to redevelop the area. In May 2020 a plan was published to convert the Citadel into a campus for the University of Cumbria. Students would learn within walls Henry VIII built to keep the Scots out.

The Citadel today is one of those buildings most travellers walk straight past. It frames the entrance to Carlisle railway station, a few yards north of the platforms, and looks like part of the station's neo-Tudor furniture by William Tite. It is not. It is older, stranger, and has held its ground while everything around it has been rebuilt, pedestrianised and re-zoned. From the south the two towers still close like a pair of clenched fists across English Street - the same line of approach a sixteenth-century traveller would have taken into a city that did not always want them. Whether the University of Cumbria succeeds in converting them or whether the towers go on waiting for their next use, they remain the most visible reminder in central Carlisle that this was once, for nearly five hundred years, the front line.