Hidden away in the pretty Kent village of Eynsford is the small but interesting castle of the same name. Built by William de Eynsford (not him personally) in the late 11th century (good old Norman), the outer wall protects the inner and outer bailey.
Due to ownership squabbles, the castle was 'vandalised' in 1312 and soon became a ruin. Briefly during the 1700s it housed stables and kennels and is now managed by English Heritage. With limited parking, the castle makes for a lovely picnic or reflection spot. Look for the Castle Hotel in the high street, the drive to the castle is opposite.
It's free to park and enter!