In 1923, Sir Iain Colquhoun sold the castle to Mrs. H. Macaulay-Stromberg, a wealthy American, who restored the castle and lived there until her death, in 1931. The castle then passed to Adelaide Parker Voorheis until 1935, when it passed to a consortium of developers who had constructed, in 1936-1937 a housing estate on what used to be the Tower Lawn. The castle then was requisitioned by the Royal Navy with the outbreak of World War II.
In 1957 most of the castle was demolished by the government in order to build naval housing for the nearby HMNB Clyde (Faslane Naval Base), though one tower was left to be used as a mount for navigational beacons and transit lights for the Royal Navy. From then on, the 45-foot (14 m) high tower was known as "Ardencaple Castle Range Rear Light", and had two green lights mounted on its south-west corner. Ardencaple Castle has been considered a Category B listed building since 14 May 1971. Today, all that remains of the grand turreted mansion is one solitary tower.
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