Colchester Castle will be hired out for parties.
Exhibits are set to be moved out of the Norman keep in January next year, to clear the way for a £4.2million revamp.
The empty venue will be hired to the public from March 9 to April 7, before it is closed to allow the refurbishments to begin.
Colchester and Ipswich Museum Services hopes concerts, plays and even a medieval feast will be held there. Members of the public will also be able to celebrate birthdays and wedding anniversaries.
Tom Hodgson, project manager, said: “We were sitting round the table and getting excited about looking at the empty castle. Someone suddenly thought, ‘we wouldn’t be the only ones who would want to see it, the public will too’.
“The last time it will have looked like this was 70 years ago. It’s a once in a two generation opportunity to see the castle as a building, not a museum.”
The museum will have a grand re-opening at Easter 2014.
Mr Hodgson, who has worked with Colchester museums since 1994, said groups would be hiring out the building and lighting, but they would have to find food and drink elsewhere.
He said: “It’s useful for us to bring in a bit of income, because we will be closed for a year. We are looking at putting a stage in there and having music concerts and theatre. I know one group is interested in it for a medieval feast.”
Tours will also be held around the building, during the day in the same month.
The overhaul was given the go- ahead last year after the Heritage Lottery Fund announced the castle had won a £3.3million grant.
Colchester Council has also given £900,000 to the project with European funding making up the rest.
Anyone interested booking Colchester Castle can call the museum on 01206 282931.