After 17 generations of occupation by the Clan Galbraith, the Napiers took over in 1632 and built a new wing to the Castle.
Seton only held the Estate for eight years before selling it to a Robert Napier, second son of the famous John Napier of Merchiston, who invented Logarithms. John Napier was an interesting character in his own right. He dabbled in the occult and wrote a treatise on alchemy.
His successors proved more law abiding, taking an interest in County affairs and serving in the Army. In 1654 the Castle was occupied for a time by Cromwell's troops.
The Napiers were an important land-owning family in Lennox as well as in Merchiston (Edinburgh) - some were Provosts of the City - as, through an earlier marriage to a Lennox, they owned estates on the Endrick right down to the shores of Loch Lomond. They lived at Culcreuch for five generations before selling the Estate to Peter Spiers in 1769.
It was a Napier who either in 1721, (the date on the stone above the Castle door), or later that century, as thought by archaeologists, enlarged Culcreuch Castle by adding the new front wing. The interior and exterior of the old castle was also altered.
The last of the Napiers, Colonel R.Napier sold the Estate for a reputed £15,200 to a Mr. Spiers from Glasgow in January 1778.