In the 12th century, the honour of Morton was a possession of Dunegal, Lord of Strathnith (Nithsdale). A ditch to the south of the castle is thought to date from this period, although it may be natural. During the reign of Robert the Bruce the lands of Morton were held by Thomas Randolph, later the first Earl of Moray. By 1307, and possibly as early as the 1260s, a castle had been constructed here, on a high defensible promontory surrounded by marshland. Randolph also constructed an enclosed deer park nearby.