These unexecuted designs for a garden fort or folly appear to show a decayed triangular castle, possibly with a moat. King has suggested that these ruins may have been inspired by the thirteenth-century moated triangular castle of Caerlaverock, Dumfries and Galloway, with which Adam may have been familiar.

Although ultimately unexecuted, Astley has explained that the purpose of this building was to provide the Child family with a contrived display of dynastic longevity: 'Having a castle in a country house park was a sign of old wealth, a visible symbol of continuity of land ownership. The Child family fortune was recently acquired; this design, which was not executed, would have given them their own ruined castle.'

There is an Adam office drawing for this ruinous garden fort or folly within the National Trust drawings collection at Osterley, and this is dated 1774 and shows the same design - albeit from a slightly different view - as Adam volume 21/2.