If someone is considering the military history of Jersey, Mont Orgueil (or Gorey Castle) cannot be missed. It virtually covers the whole history itself!
It is clear that the Gorey promontory where the castle now stands has been occupied from the Neolithic period, if not the Mesolithic period. Iron Age defence ramparts have been discovered within the walls of the present castle proving that the promontory was a place that the local population would have sought refuge and defended.
Major medieval castle construction commenced in the first few years of the 13th century and continued for the next 400 years or so as bows and arrows were replaced by cannons and guns as the weapons of war. Down-graded in importance by about 1600, it nonetheless continued to house a small garrison, particularly in times of war, and served as the main prison until one was built in St Helier.
In the French Revolutionary Wars and Napoleonic Wars, it served as the centre of the La Correspondence, a spy network operating in France.
Whilst the Castle then ceased any military use and became an ancient monument visited by locals and tourists alike, it nonetheless housed a militia guard in the First World War and was then occupied by the Germans in the Occupation, who added some of their own concrete additions still very evident today if you look closely.