Its history is chequered with sieges, murders and intrigue.
Award-winning cave tours tell this history and take you down into the passageways and tunnels beneath the building.
Whilst on your visit you can also enjoy the six acres of Victorian inspired grounds and gardens.
The grounds boast magnificent views of the city and beyond.
Police said they had identified at least 60 companies involved in the refurbishment works and were seizing documents in relation to the criminal investigation into the fire.
“We will identify and investigate any criminal offence and, of course, given the deaths of so many people we are considering manslaughter, as well as criminal offences and breaches of legislation and regulations,” Detective Chief Superintendent Fiona Mc Cormack said.