Robert de Clifford, 1st Baron de Clifford, of Appleby Castle, Westmorland, feudal baron of Appleby and feudal baron of Skipton in Yorkshire, was an English soldier who became 1st Lord Warden of the Marches, responsible for defending the English border with Scotland.

Appleby Castle is in the town of Appleby-in-Westmorland overlooking the River Eden. 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.