You HAVE to come here if you go to Oslo. Greta views from the top. Free to get in. Wander around and get lost for an hour. Be careful with the statues, they can be scary!
When i visited the Castle they had actors dressed like in the Middle Ages explaining about habits, food and games from the past. They told me this game was popular during the viking era.
Features views of the Fjord, a cute fortress and a corny WWII museum. A great walk in itself, rarely crowded, potentially a spot for soaking up sun (on the Western embankments - don't fall off.)
Absolutely a worth visiting place. It used to be a medieval castle but was remodeled into renaissance style in the XVIIth century. There beautiful views of the sea there as well.