Tower houses across Scotland followed a fairly standard layout, but Plunton breaks the pattern in one specific way. Its ground floor is divided into two tunnel-vaulted storage chambers - and the two chambers do not connect to each other. The southern one is reached through the stair wing; the northern one can only be entered from the courtyard outside. Maurice Lindsay, who directed the Scottish Civic Trust, called this arrangement 'defensively weak.' John R. Hume, former chief inspector of historic buildings for Historic Scotland, called it 'extremely rare' and a 'breach of security.' The Lennoxes addressed it partially by enclosing the whole tower in a walled barmkin about 2.74 metres high - the wall is gone, but the stonework on the north-west angle shows where it once joined. Splayed gun loops in the gable walls of each ground-floor chamber gave defenders inside a wide field of fire.

The site itself does much of the defensive work. Plunton sits on a shallow rise, but it is well protected by Plunton Burn on the west, a steep ditch to the north, and marshy ground on the other two sides. An attacker would have had a wretched time getting close enough to bring weapons to bear. The tower is built mostly of local greywacke rubble, with dressed sandstone around the doors and windows - the sandstone is largely gone now, taken for other buildings after the castle fell out of use. The main block is 9.1 metres by 6.5 metres, with walls about 1.1 metres thick, and stands 10.7 metres to its full height. A projecting wing at the south-west corner houses a turnpike stair.

Climb the turnpike stair and the first floor opens out into what would have been the castle's great hall. The plain fireplace in the middle of the west wall is about 1.8 metres wide, with a corbelled-out lintel and a stone cupboard - an aumbry - tucked to its left. Windows in all four walls would have admitted light. The original iron grilles are gone, but the fittings tell you they were once shuttered behind fixed glazed panes - sophisticated detailing for a Galloway laird. The second storey was divided by gable fireplaces into two heated chambers. The attic above, also divided into two, gave access to turrets at three of the four corners, each supported by corbels and a row of dentils. Each turret has gun loops angled downward, so defenders could shoot anyone trying to approach the outside walls.

In 1455, when the king's feud with the Douglas family ended in their lands being forfeited, the crown divided Plunton into two parts. King's-Plunton went to the Lennoxes of nearby Cally; Plunton-M'Gee, originally owned by the McGhie family, was acquired by the Maclellans of Bombie. By 1575, Andrew Lennox of Plunton had reunited the whole estate, and the castle most likely went up around his marriage to Janet. After that the property passed by marriage to the Murrays of Broughton in the late seventeenth century. A 1654 map attributed to Timothy Pont and published by Joan Blaeu shows the castle surrounded by a fenced-in deer park. An Iron Age Celtic armband was found near the site in 1826 and now sits in the National Museum of Scotland - the place has been somebody's home for far longer than Lennox or Murray.

Walter Scott visited Plunton and was struck by its romantic setting. He used it as the model for the dungeon-haunted ruin in his melodramatic play The Doom of Devorgoil. The play flopped - it remains one of Scott's least successful works - but the connection has kept Plunton on the literary tourist map. The castle was designated a scheduled monument in 1937. Historic Environment Scotland describes its condition as fragile, but they note that enough survives to permit a faithful restoration, as has happened at nearby Barholm Castle. As of 2021 the current owners had considered several restoration proposals; none has been started. The tower waits in its field, walls to the wall-heads, gun loops still aimed at imaginary attackers, the wind blowing through where the roof used to be.