Comlongon Castle is regarded as one of the finest preserved examples of a Border stronghold. It is built of pink dressed sandstone, with a rubble insert, on a wide plinth base, to act as a strengthener on what was once marsh ground. Measuring 50 feet square and standing 70 feet high, it retains many original and unusual features.

The Great Hall, used as a meeting, living and banquet room, would have originally been plastered and painted with murals in vivid prime colours, depicting historical, religious and heraldic themes. Around the walls at ceiling level are some fine examples of stone carved corbels, bearing arms of families connected with the Murrays through intermarriage. Above the fireplace is an early 15th century royal coat of arms. All these carvings were at one time highlighted with coloured paint.

The western side of the hall is dominated by the huge open fireplace. An original lintel (long since gone), was replaced by a wooden beam unequal in length, propped up on one end by a carved pillar. Above the lintel is an unusual carved overmantle depicting various plants and heads emanating from a dragons breath, believed to depict the ancient Celtic legend of the green man, or god of spring. To the right of the fire is a small dressing room where the laird would have changed into a variety of outfits, to entertain a constant stream of business delegations and social guests.

The castle, built on the tidal plains of the shallow Solway estuary to combat the frequent Border incursions by English raiders, is mentioned as early as 1331 when the Earl of Moray, Thomas Randolph, gave the property to his nephew, Sir William Murray.