Jersey is one of the Channel Islands which are situated closer to France than England. Guarding the western approaches to the English channel they have always been of military importance. They have been occupied by, on occasion, the French (Napoleonic) and Germans (WWII).
Elizabeth Castle at St. Helier. Built in the late 16C. At low tide you can walk to it, at high tide you have to swim or take the amphibious bus (Puddle Duck). There is, daily, a recounting of the Napoleonic 'Battle of Jersey' by a suitably dressed presenter (this was a skirmish in the main square of St. Helier). Very much an audience participation event, including 'drill', which also includes a demonstration on bayonet practices, musket shooting and the firing of a cannon at 1p.m. (?) - to tell the French the island is still ready for them. It has a small military museum with exhibits from the Napoleonic Wars to WWII.
Mont Orgueil Castle (more often called Gorey Castle) was the main castle on the island until Elizabeth Castle was built during the reign of (you guessed it) Elizabeth I. It is sited on top of a high mound (it looks like it is perched on top of a volcano). I'm afraid my distance shots did not come out, but it looks very imposing in a medieval kind of way; it is a very steep climb to the top. For a medieval castle it is in excellent condition. It includes the later addition of a 16C gun tower (built to defend against opposing artillery which could be sited on an overlooking hill), a very nice shooting gallery (North East Passage) as well as some truly awful German WWII additions. They also provide free (to wear on site) helmets, shields and swords for kids (see my son demonstrating the 'double parry' defence).
The Channel Islands Military Museum at St. Ouen's Bay is a small one. It is situated inside a WWII German coastal defence bunker. It is worth a short visit; and is a great place to pick up a holiday souvenir (I got my vintage 20mm Flak 38 shell cases here - £14 the pair).
The maritime museum also has a few exhibits. A U-Boat gun and various other odds and sods, including the mast (or what is left of it) of a US torpedo boat that bit off more than it could chew.