It was built for Archibald Acheson, 2nd Earl of Gosford, and designed in the Norman revival style by London architect Thomas Hopper.
In 1837 the building was reported as unfinished, and was not fully complete until 1859 when Hopper's assistant George Adam Burn rebuilt the entrance front.
[3] Following the war the Achesons sold the estate to the Ministry of Agriculture, who established the 240 hectares (590 acres) demesne as Gosford Forest Park.
The castle at this time was in a desperate state of disrepair and without urgent intervention was in real danger of becoming irretrievably beyond repair, leading to calls for action from the Ulster Architectural Heritage Society (UAHS).
The developers, supported by architects the Boyd Partnership and the Environment and Heritage Service, put forward a £4m proposal to restore the castle as 23 residences, based on vertical division of the building and retention of the main rooms.
Approaches used include the restoration of original colour schemes in the principal rooms, such as the vaulted ceiling of the Inner Hall; walls that were painted green to represent drapery; and a background of scarlet used to set off the library's wooden bookcases.