As suggested below it's just a ruin but worth a look around the outside at least. As it's part of the English heritage sites you are admitted with a discount.
There isn't any parking but plenty in and around the town. The castle itself is just off the Market place. If you walk down the hill past the entrance you can get a good perspective of this once mighty fortress & some good views of the river Tees & medieval bridge that is still used today.
There are some nice walks on the other side of the river in either direction with foot bridges not more the half a mile to return you back to the castle.
Probably not the best looking castle in SW Durham but it is up against Raby, Witton, Walworth, Brancpeth, Auckland & Durham itself. Worth a visit though if you are in Barnard Castle for day out. read more