This (free) museum is situated inside the fortress. Since everything was so expensive in Oslo, we stuck to the free stuff and ended up there. It was great! We learnt a lot about military history, the wars with Sweden and Denmark. Even my sister, who's usually not into history, enjoyed it, it was very well presented.
But we also had such fun in the contemporary "galleries", where a tank or a cabin of a battle ship are exhibited. it was a great way to get closer to the armed forces and understand them.