Like Wolvesey Castle in Winchester, Farnham Castle started life as a defensive structure but later became one of the many palaces of the medieval Bishops of Winchester.
Only the old keep is open to the public having been put in the guardianship of the State in 1933. It is now managed on behalf of English Heritage by the owners of the adjoining Farnham Castle Training and Conference Centre based in the old Bishop's Palace, a complex of Grade I and II listed building.
Please see the English Heritage web site for details of opening times, etc.
Originally built to a motte and bailey design in around 1138 by Henry of Blois, Bishop of Winchester and brother of King Stephen, it was extensively modified to a shell keep design about a hundred years later.
Unusually, the builders built the curtain wall so that it enclosed the old motte, rather than around the top of the mound. As a result from the outside these walls are really impressive.
The interior of the keep, however is disappointingly empty, as presumably most of the interior buildings were wooden.
The only substantial structure is the base of the original square keep, which was later used as a well. There are steps down to a viewing platform inside the old tower.