Carlisle Castle guards the western end of the border between England and Scotland. William II built the first timber castle at Carlisle in 1092, and thirty years later his brother, King Henry I, ordered the building of a castle in stone which included the keep that now remains as the oldest part of the castle. The keep and castle walls were eventually completed by the Scottish king, David I, who took control of northern England during the troubled reign of King Stephen (1135-54). By 1157 the castle was back in English hands, but its location at the border between two frequently warring nations meant it would be battled over for many more centuries. The last time the castle came under siege was in 1745 when the Jacobite forces of Prince Charles Edward Stewart (Bonnie Prince Charlie) successfully captured the castle, only to be forced to surrender the following month.

The oldest surviving buildings, other than the keep, are the inner and outer gatehouses, both built in the 1160's and substantially altered in the late 14th century. Most of the other medieval buildings have been lost under the 19th century barracks and other army buildings that line the inner and outer bailey. The castle remains the headquarters of the King's Own Royal Border Regiment and it houses the Regimental Museum.