Mont Orgueil Castle. Last one for tonight, here's some history for ya:
It was built somewhere around the time of 1212 and was Jersey's primary defence until gunpowder was invented. It was then replaced by Elizabeth Castle (near St Helier, south of the Island) and Mont Orgueil/Gorey Castle was used as the island's only prison until the end of the 17th century when one was built in St Helier. It was 1693 when the castle was declared to be in a ruinous condition and abandoned as a prison. Repairs were carried out in the early/mid-18th century. Until the mid-19th century the castle was only open to the public once a year - on Easter Sunday (it got really busy). It has been a museum site since 1929.
It's a good day out and you get awesome views from the top but it has a lot (and I mean a lot) of stairs, so not so great for people who struggle walking.