It was the official residence of the Bishop of Worcester for 161 years.
Campaigners hoping to stop a castle from falling into private hands are facing increasing pressure to raise enough money to save it.
Hartlebury Castle Preservation Group needs £2.5m to buy the castle but have not yet reached £1m.
Alison Brimelow, chair of the Hartlebury Castle Preservation Trust, says the next few months are critical:
"I think we could get there (but) I believe we are moving into last chance saloon now."
The Church of England wants to sell Hartlebury Castle, the official residence of the Bishop of Worcester from the 19th century until 2007.
The group are in the process of bidding for a Heritage Lottery grant to add to their funds.
Partnering with the Worcestershire County Museum - housed in the north wing of the Castle - could also be an option.
The Great Hall at Hartlebury Castle.
Alison Brimelow says competition for grant money, and private donations, is fierce:
"If you compare us with somewhere like Croome Court, which is an appeal that's running at the same time, is owned by the National Trust, and has a future that you can see - if you put money to support Croome, what will happen will be something you can understand and see.
"Hartlebury Castle is still a relatively unknown quantity, so if you give money you are not going to be entirely clear what you are giving to - other than that you are trying to save the castle for public use in Worcestershire."
The group's launched a series of fund-raising schemes because they say the financial climate has meant the number of private donations have dropped.
Small donations have helped, according to Alison Brimelow, but they won't help them reach their target in time:
"There's a steady stream of modest donations, which are very important and very helpful to the campaign, but it is not coffee mornings that raise £2m."