The castle is undergoing an improvement programme, and its owners, English Heritage are taking the opportunity to break through into some rooms which were sealed for safety reasons as the building crumbled into disrepair more than a century ago.
It is now believed one of the rooms was converted to a bathing room for owner Sir William Cavendish, with a boiler room alongside.
English Heritage spokesman John Burditt said: "We are trying to improve visitor access to parts of the castle and to further our knowledge of the site at the same time.
"Some doorways have been opened up which have not been open for more than 100 years.
"We are quite excited because we think one room is a bathing house.
"At some stage in history, bathing was invented and this was probably after the Civil War, in the 1660s. We are also looking at the back, into a smaller chamber which may have provided hot water."
The archeological work is expected to continue all year and may lead to permanent access for visitors to new areas.
The castle was built as a weekend retreat by Sir William Cavendish after his father Sir Charles started work on the site in 1614. Although it has the appearance of a medieval castle, it was built much later.