I tend to like any types of castles that look old, so I might not be the most objective person here, but I really liked, even I chose not to enter any of the museums (limited time in Oslo, wanted to see some more things). the walk along the grounds (that is free) turned out to be longer than I had expected due to the high level of enjoyment I had.
It has it all - panorama with wild coast, panorama with sea, panorama with modern part of city, panorama with city centre. And ships and boats moving, that I am sure any child would be excited to watch (those present there were for sure).
And it has also that old-castle quality, that makes at least me want to know more about the time it represents for the respective country/place. It felt like walking along a city in a city with its own atmosphere, because it felt quite isolated from the rest of the Oslo.
And yes, due to the presence of military school there, there was quite a number of people walking around in miltary uniforms, including the honorary guards that, having just arrived from their duty at the palace, did their military ritual to return to their department, marching elegantly. Even better than watching the guard-change at the palace.
Visitors centre with their exhibition on fortress as prison was also interesting - especially the stories of master-escapees.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.