The fire at Lincoln Castle which damaged two medieval doors is being treated as suspicious by police.
Lincolnshire Fire and Rescue and the police were called to the incident at the Lucy Tower entrance in the early hours of this morning.
The fire was extinguished using two water backpacks and police believe an accelerant was used to start the blaze.
Officers searched the grounds of the castle with the aid of a police dog but no intruders or trespassers were found.
Investigations into the incident are continuing this morning and police are appealing for witnesses.
James Newall, spokesman for Lincolnshire Police, said: "We would like anyone who saw anything suspicious in the Union Road or Westgate area between 3.30am and 4am this morning to get in touch.
"This is being treated as a suspicious fire."
Anyone with information should call Detective Constable George Wynn of Lincoln CID on 101 quoting incident number 29 of June 11.
Rachael Thomas, castle development manager, said: "It's disappointing that somebody would want to do this to the castle.
"Thankfully, the fire was quickly extinguished, but there was damage to two doors leading up to Lucy Tower.
"We're now assessing the damage to see whether they can be repaired or need to be replaced.
"In the meantime, we'll be helping the police with their investigations."