The castle itself is a white building that was built 1634 and the construction has been altered after that. First it was royal owned but later in 1792 they started up the Military Academy and it still is, so a fun fact is that it's the oldest military academy in the world.
The area is guarded with gates etc but during daytime its open for visitors in the park and you can pass by in front of the castle anytime during the day or night.
View to the castle from the park
It's a quite spacious park with good tracks for walking or running
Close to the city parts of Vasastan and Kungsholmen
The canal of Karlberg (Karlbergskanalen)