'Auchans Castle' or 'Old Auchans', is a mock military mansion, Category A listed. The spelling on Joan Blaeu's map of 1654 is 'Aghans'. As a T-plan building of a late 16th century date, it was converted to L-plan during the early-to-mid-17th century and was held at various times by the Clan Wallace, Clan Cochrane and Clan Montgomery families.

Auchans has been referred to as being amongst Scotland's principal châteauxs, defined as the ''dwelling of the owner of  a great property, a large and beautiful pleasure house in the countryside'', and records that James Wallace added the fashionable square stair-tower with its viewing platform and broken pediments in 1644.

The castle stands on a slightly elevated knoll and is constructed mainly of whinstone. The design is reminiscent of the old Glasgow College buildings, long since demolished. The original house, with its high gables, had three principal storeys; the basement was vaulted and a stair-tower was situated externally, central to the South side.

The ballustraded terrace on the South side of castle was at one time enclosed within a courtyard. A new wing, three storeys and an attic in height, was at a later date added at the West end of the North wall, and a stair-tower, with a Renaissance-style doorway, was built. This new wing was extended still further on the North by the addition of a block with two towers. The basement of the block was vaulted; one of the towers was corbelled, square in section and its gables crow-stepped. Domestic buildings were added on the South and East sides of the courtyard.

The kitchen stood in the western wing; the first floor contained a suite of principal rooms, that in the western wing being known as the dining room in the 1860s, probably originally being the private room and bedroom of the proprietor. It was wood panelled and had an ornate marble fireplace.

The second floor contained bedrooms and the third floor, partly in the roof, was chiefly occupied by a long gallery. This was lit by dormer windows and by a large traceried window in the east gable, long built up. The entrance was in the square balustraded tower and was of the Renaissance style.

The main block of the castle was not vaulted and only the cellars in the North wing had vaults. Only a few gun-loops were provided as by 1644 such defensive structures were largely redundant.

The now greatly ruined castle stands in its woodland policies amidst a series of stone-walled parks, the walls of which are mainly in a state of collapse. The building and the park walls were in the main constructed using stone robbed from Dundonald Castle.

A vast number of valuable Eglinton family papers were discovered in one of the apartments in the 1880s, rescued as the building was in a terminal state of decay. Many had already been destroyed through neglect.