It's a small but well-designed place dedicated to the history of military communications, from Ancient Greece to today. You can play with a model shutter telegraph, marvel at the stuffed remains of an heroic WWI pigeon who got his message through despite being mortally wounded, expiring shortly after delivery, and telephone your friends in other parts of the museum (press 1 for the Crimean War, 2 for Northern Ireland).
cybersofa wanted to bone up on the D-Day exhibits before his trip to Normandy next month, and I to see the Cryptography display, though most of it turned out to be recycled from 'Secret War' at the Imperial War Museum six years ago.
It's always fun to visit, since the museum is situated amongst a real actual army base and thus you need to give your name and address, be photographed and issued a temporary pass before you can go in.
Given that the bloke on desk duty thought I was (a) male and (b) eligible for an under-14's ticket, however, you could probably wander in quite safely with several kilos of Semtex strapped to your person.