Six fire crews and a height vehicle tackled the blaze at the empty property, once owned by the family of the Scots king, with firefighters still present until 11am to dampen down any hot spots left.
There were no reports of injuries.
The once popular wedding venue, which overlooks the village of Airth and the River Forth, operated as a 125-room hotel and spa until it went into voluntary administration and closed its doors in March last year.
Its owners said it had raked up ‘unmanageable debts’ following the Covid pandemic and rising energy bills.
In June, police increased patrols around the castle amid increased reports of break-ins, fire damage and vandalism.
And yesterday the force confirmed that following inquiries into the early morning blaze, the case is now being ‘treated as wilful’.