Oslo is not only a modern city, but it also has a medieval fortress right at the eastern side of the harbor. It is very strategically located, after all, this structure dates back to 1299 when the king ordered the fortress to be built so as to protect the city from threats. Nowadays, it doesn't have that role anymore, but nevertheless the military still has some buildings inside the complex. The yards are free to stroll around, which is what we did when we visited.
There are several museums that one can visit here, but nevertheless, it is also perfectly fine to just walk around and admire the view.
I thought I liked the fact that it showcased a wide variety of architectural styles throughout the ages. From the stone walls to the buildings, one can see that this place underwent plenty of renovations and rebuilding efforts as centuries passed. Nowadays, it looks very peaceful and acts as event venues, but it definitely has a military history, and in some areas, you can still see the military offices.
It's a nice place to stroll around. If you want, you can climb up to the walls and see the surrounding area. It provides a very good view of the harbor as well.