The keep, or great tower, is the central or focal building of Carlisle Castle, dominating its inner ward and commanding the stronghold’s skyline. Built of sandstone, it is a square palace-keep of classic Anglo-Norman type, and is now four storeys high with a roof platform. 
An English building or a Scottish one?
We do not know exactly when it was built, and the building has been too much altered to be dated from its architectural details. Henry I ordered Carlisle to be fortified with a ‘castle and towers’ in 1122: he built similar square keeps in several other places, but it is not clear how far work had advanced by the time of his death in 1135.
The Scots occupied Cumberland in that same year, and one English chronicler says that David I of Scotland built the ‘mighty keep’ ('fortissimam arcem'). David I died there in 1153.  Henry II retook Carlisle and Cumberland, and repaired the castle, though the sums of money recorded in the Pipe Rolls do not suggest anything was built on this scale.
So it is possible that the keep was begun c 1122–35 – that is in the reign of Henry I – but that it was completed, or rebuilt from scratch, by David I of Scotland during the Scots occupation of Cumberland. If so, it has a unique distinction as the only Anglo-Norman keep built by a Scots king, and the only English castle which has been a royal residence for kings of both England and Scotland.
The keep has been much altered, and is of great archaeological complexity, so its original form is not altogether clear, but it probably had three storeys with a countersunk roof concealed within the upper part of the walls. The foundations of an entrance fore-building, demolished in the medieval period, were revealed by excavation in the 1930s on its eastern side.
Inside the keep
The keep has a spine wall, dividing each floor into two main rooms, as do most Anglo-Norman keeps of its size. The story at Carlisle is more complicated, however, for the spine wall’s masonry is later, probably dating from von Haschenperg’s works in the 1540s. Furthermore, it seems to overlap or block earlier features in both the north and south walls. Dr John Goodall has established the probability that the keep, in its original form, had a single large room on each floor. 
The 16th century work also included the removal of the original roof, the lowering of the walls, the insertion of a new third floor, and the construction of a new, vaulted roof to carry a gun platform. The keep may initially have had corner turrets. If so, they were removed and a new parapet with openings for cannon built, markedly altering the building’s skyline. The date of this work is unknown, but probably post-dated 1580.
The ground floor has an entrance lobby leading down into a passageway, from which three vaulted rooms open. These were used as a prison in the 18th century, and possibly earlier. An apparently primary spiral stair in the north-west corner leads to second-floor level. A mural staircase in the south wall, also primary, leads to the first floor.
This has two large main rooms and a number of small mural chambers. The second floor has two large main interiors, and several mural chambers: Room 22 contains a remarkable group of so-called ‘Prisoners’ carvings’ of c 1480, whose significance remains unclear (see below). The third floor was inserted in the 16th century, and is roofed with the brick vaults referred to above. A narrow timber stair gives access to the roof platform.
On the second floor of the keep, a short passageway gives access to two mural chambers. In this area is a remarkable group of carvings, whose full significance remains unclear. Some of them are graffiti, being little more than scratches, but several of them are finely carved, and may actually be the work of a single hand.
They are in relief, slightly recessed into the stonework, in no particular pattern or order. They used to be referred to as ‘prisoners’ carvings’, but this area is not known to have been a prison, and they do not seem to make much sense in this light.
The heraldry, armoury and other details indicate a date in the 1480s for the group. The emblems carved include the white rose for the House of York; the boar emblem for the Lord Warden, Richard, duke of Gloucester; the scallop and dolphin emblems of the local Dacre family; and the fetlock emblem of the Percys.
There are several religious badges, include a crucifixion, the ‘IHS’ or name of Christ, and a figure with a wheel, who may be St Katharine. The carvings seem more likely to be members of the castle’s garrison or household, expressing loyalty to the Lord Warden and to the great local families.
Goodall, J A A 2004. ‘The Great Tower of Carlisle Castle’ in M McCarthy and D Weston (eds), 'Carlisle and Cumbria: Roman and Medieval Architecture, Art and Archaeology', British Archaeological Association Conference Transactions, 27, 39–62, Leeds.
Brown, R A, Colvin, H M and Taylor, A J 1963. 'The History of the King’s Works, II – The Middle Ages', 595–6, London.
McCarthy, M R, Summerson, H R T and Annis, R G 1990. 'Carlisle Castle: a Survey and Documentary History', English Heritage Archaeological Report, 18, 75–8, 82, London; Goodall, J A A 2004. ‘The Great Tower of Carlisle Castle’ in M McCarthy and D Weston (eds), 'Carlisle and Cumbria: Roman and Medieval Architecture, Art and Archaeology', British Archaeological Association Conference Transactions, 27, 45, Leeds.