The Norwegian Defence Estates Agency manages 14 national fortifications all over Norway. They welcome a total of 2.5 million visitors a year.
Karljohansvern is in Horten, on the western shore of the Oslofjord, a good hour's drive from the capital. The fortress is beautifully situated on a peninsula between the fjord and the inner harbour and consists of both big storage buildings from the mid 19th century and ordinary houses from the 20th century. There are also nice recreational areas along the fjord.