Englands only double moated Norman castle dating back to 1066. With links to Thomas Becket, King John of France and the Black Prince, Berkhamsted Castle was once a noteable fortification. Sadly it has fallen into ruin, but an interesting few hours can be spent wondering at its motte and bailey. Whats more its free to visit
More info from the English Heritage click here

Although the house is now a business school parts of the gardens at Ashridge are open to the public at weekends only, between Easter and September.There is a little information by clicking here and a map by clicking here
There are also fantastic walks here and nearby in National Trust land