The 15th-century Ackergill Tower, in Caithness, which plays host to exclusive events and guests and was the location of Philip Schofield's wedding in 1993, suffered smoke damage to all five of its storeys in the fire.
The blaze broke out in the early hours of yesterday morning at the grade A-listed building. It is thought a member of the public alerted the fire service after smoke was seen coming from the premises.
A fire service spokeswoman said a call was received at 4.23am and two crews dispatched from Wick to the castle, where it took over an hour to control the fire, which completely destroyed the cellar.
There was no-one in the building at the time of the blaze.
Police Scotland were appealing for anyone with information or who may have seen someone acting suspiciously in the area to contact them.
Ackergill Tower, a listed building and landmark in Caithness, is owned by Clarenco, a group whose founder and chairman is Mike Clare, who made his millions at the helm of former bedroom furniture company, Dreams.
The exclusive venue, which has 35 bedroom suites, is also used for corporate hospitality events and as a film location and celebrity bolthole.
Meanwhile, four people have been taken to hospital following a fire police suspect was started deliberately.
Fire crews attended the blaze at a block of flats on Westfield Road, Edinburgh, at around 12.30am yesterday morning.
A Police Scotland spokeswoman said: "Four people were taken to hospital as a precaution with some minor smoke inhalation. The fire originated in the common stair of the block, which has 16 flats in the property."
A Fire Scotland spokeswoman said: "Crews from Sighthill and Tollcross attended a fire in a cupboard in a common stair in a block of flats.
"Three females and one male were taken to Edinburgh Royal Infirmary suffering slight smoke inhalation."
She added: "It appears it was possibly started deliberately."
A Police Scotland spokesman said: "Fortunately, no-one was seriously injured as a result of this incident and officers are now conducting local inquiries to identify whoever was responsible for starting the fire.
"Anyone who remembers seeing anything suspicious in the Westfield Road area in the early hours of Sunday morning should contact police immediately."