Don't get too excited about visiting the castle, it's strictly private. You won't even be able to see it, except perhaps some turrets across the fields from GZ - but you can find lots of pictures online. I have placed this cache partly as a means of transporting TBs up and down the M40, which is 2 miles away, but also to draw people's attention to nearby Shirburn Castle, which lies hidden behind the walls. Even most people living locally have probably never seen it!
Shirburn Castle was built in 1378. It is a square-ish building with a round tower on each corner, and a moat and drawbridge. It reminds me a bit of Bodiam Castle in Sussex which was built about eight years later, but it seems to be quite a bit smaller than Bodiam.
In 1716, Thomas Parker, first Earl of Macclesfield bought the castle. It was remodelled in 1720 and alterations were also made in the early 19th century. It is still owned by the same family - or rather a family company set up in 1922 "for tax reasons". What might have seemed a good idea at the time has turned out to be disastrous for the castle, as it has resulted in a very bitter family feud. The current earl moved out in 2004/5 and the castle has been empty ever since. The contents belonged to him and a number of them were sold at auction, in particular books and letters found in the castle's library. Tragically, it would appear that neither side is willing/able to take responsibility for the upkeep of the castle and it is apparently in desperate need of repair.
The cache is placed next to the road to Pyrton, across the fields from the castle. If you're coming from the M40 (J6), you need to turn right at the crossroads between Shirburn and Watlington. You can pull in to the side of the road right next to the cache. It will take you about 5 minutes to get here from the motorway. If you're on a long journey with children, I recommend taking a break at the newish playground a bit further along the main road, where there is some really fun equipment for children of all ages, plus picnic tables. There are shops and pubs in Watlington and some very pretty old buildings.