Discover some of Oslo’s most important history at this medieval fortress and castle overlooking the city’s fjord approach. Built in the late 1290s by King Haakon V to protect the city from invasion, the complex has been in continuous use ever since, including as a prison and military site. A quick stroll around the free museum—not to mention the Norwegian Armed Forces Museum and Resistance Museum—is enough to transport you back in time. Watch out for the smart young soldiers patrolling the grounds. They’re not there to entertain tourists—the fortress is still an active military site.