The fire broke out around 1:30 a.m. Tuesday at the Rocky Hill Castle on the 100 block of Bullock Road.
Firefighters had difficulty battling the blaze because it is located in a wooded area.
Chadds Ford crews called for additional water tankers from Chester and New Castle counties.
The castle was built almost 200 years ago, in 1821, by a business owner who used the property as a summer home.
The current owners bought the castle in 1957.
It has recently been listed for sale for $674,000, and reportedly in need of renovation.
There were no immediate reports of injuries.
Fire investigators are working to determine what sparked the blaze.