This obligation was carried out in good faith by Hugh of Padinan, progenitor of the Iloustons. Crawfurd relates that Hugh erected a stone castle, "extensively and strongly built as befitted the power of the chiefs" of Houstoun. This structure, added to from time to time, endured for 600 years. During the reign of Alexander III (1249-1286), it was rated as one of the greatest fortresses in the country.

The castle, when completed, was in the form of a square with a spacious courtyard inside. The outer walls were of solid masonry, six feet thick. Surrounding the walls was a moat or large ditch, sometimes filled with water.

On the southern front were two turrets, between which was the grand entrance to the area, arched above and secured by a portcullis (suspended iron gate). A drawbridge across the moat secured the approach. On the northwest corner was a large, tall tower, described as the most ancient part of the structure. A lower building was joined at the east end of the tower "with vaults below and a very wide and long paved hail above, with antique windows in front."

The ceiling of the great hall was unplastered, but the timbers of the roof were made of massive oak, gracefully arched. Other additions made from time to time completed the square.

"The building was a real fortification, being large and built on an elevated situation,." writes Campbell. "It had a lordly appearance, overlooking the whole plain toward Paisley and Glasgow."