Appleby Castle is in the town of Appleby-in-Westmorland overlooking the River Eden (grid reference NY685200). It consists of a 12th-century castle keep which is known as Caesar's Tower, and a mansion house. These, together with their associated buildings, are set in a courtyard surrounded by curtain walls. Caesar's Tower and the mansion house are each recorded in the National Heritage List for England as a designated Grade I listed building. The uninhabited parts of the castle are a scheduled ancient monument.
The castle was founded by Ranulf le Meschin at the beginning of the 12th century. In about 1170 the square stone keep known as Caesar's Tower was built. The castle was in royal hands when the Scottish king, William the Lion, invaded the Eden Valley in 1174. The constable of the castle surrendered without a fight.
In 1203 the castle was granted to Robert I de Vipont by King John. In 1264 it came into the possession of Roger de Clifford, through his marriage to Isabel de Vipont, one of the two daughters and co-heiress of Robert II de Vipont. Appleby Castle remained for nearly 400 years in the ownership of the Clifford family, who were responsible for much restoration of the castle. Roger's son, Robert de Clifford, inherited the castle in 1282.
The north wall of house and the west part of north wing with the round tower date from the 13th century. The eastern part of the house was built in 1454.
In the mid-17th century, Lady Anne Clifford made the castle her home. The castle was partly dismantled following a siege by roundhead forces in 1648, during the Second English Civil War. However, it was restored by Lady Anne Clifford in 1651–53. On her death the castle passed to the Earls of Thanet. They were responsible for converting the hall block into a classical mansion house. The upper parts of Caesar's Tower were altered in the 17th and 18th centuries. The house was largely rebuilt in 1686 and the northwest wing was added in 1695. In the 19th century it was again restored and sash windows were inserted.