Helmsley Castle, Yorks.

English Heritage.
The castle, originally a medaeval fortress, later acquired a luxurious Tudor house within its walls. The castle was rendered unusable at the end of the Civil War. Later still, it was regarded as a Victorian romantic ruin.
Today, the circuit of walls is largely intact. Inside is a wide area of grassy lawn. The visitor enters via a gatehouse and can look at the half of the keep still standing, the other half having been blown up and allowed to fall into the moat. The Tudor residence is now used as an exhibition space. An audio guide is available.
The castle is definitely worth a visit if you are in the area. Other EH properties are nearby.