Members of the public used to be able to visit for a small fee but have been unable to do so for almost a year as it “wouldn’t be safe”, Ms Knight said.
Its Franciscan gardens remain open.
The clerk and receiver, who has held her role for eight years, said it was “incredibly sad that the front door shut”.
“It’s been an uphill struggle trying to keep on top of all the maintenance just because we’re talking in the hundreds of thousands,” she said.
“It’s hugely important to keep the building going and it’s important, I think, for people to be able to come in and understand the history.”