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The castle dates back to 1093 when William II of England started construction on what was the site of a Roman fort. The prime function of the castle was to protect the northwest border of England from the recently deposed Scots.

The present stone keep was built between 1122 and 1135, at the same time as the city walls.

The Scots managed to take the castle by storm on numerous occasions over the following centuries, most recently during the Jacobite rebellion of 1745, which was the last time the castle saw active service.

The Army took hold of the castle, which became the regimental depot of the Border Regiment until 1959, when this Regiment amalgamated with the King's Own Royal Regiment (Lancaster) to form the King's Own Royal Border Regiment. The King’s Own Border Regiment Museum is housed within the castle.

The building is under the care of English Heritage and is open to the public.

Listed buildings and structures are officially designated as being of special architectural, historical or cultural significance. There are over half a million listed structures in the United Kingdom, covered by around 375,000 listings.
Listed status is more commonly associated with buildings or groups of buildings, however it can cover many other structures, including bridges, headstones, steps, ponds, monuments, walls, phone boxes, wrecks, parks, and heritage sites, and in more recent times a road crossing (Abbey Road) and graffiti art (Banksy 'Spy-booth') have been included.

In England and Wales there are three main listing designations;
Grade I (2.5%) - exceptional interest, sometimes considered to be internationally important.
Grade II* (5.5%) - particularly important buildings of more than special interest.
Grade II (92%) - nationally important and of special interest.

There are also locally listed structures (at the discretion of local authorities) using A, B and C designations.

In Scotland three classifications are also used but the criteria are different. There are around 47,500 Listed buildings.
Category A (8%)- generally equivalent to Grade I and II* in England and Wales
Category B (51%)- this appears generally to cover the ground of Grade II, recognising national importance.
Category C (41%)- buildings of local importance, probably with some overlap with English Grade II.

In Northern Ireland the criteria are similar to Scotland, but the classifications are:
Grade A (2.3%)
Grade B+ (4.7%)
Grade B (93%)

Read more at Wikipedia Link