Here's an interesting topic: Nations keeping secrets.
Using a medieval age for starters... let's say one side has much better designs of war ships. How do they keep their enemies from obtaining a ship or a traitorous ship builder? I remember hearing the Romans had to find a Carthaginian shipwreck, before they learnt how to construct ships like the Carthaginians'.
How do nations like Carthage make their ship designs so hard to obtain?