• Free kids' activities for ages 4 to 15 whenever the museum is open, such as drop-in adventures, a trail and drawing workshops
• Open in summer only, mid-May to early September, plus some school-holiday weekends; in winter just for events and booked groups
• Kastelletbåten from Vaxholm crosses every 20 minutes in season, and some Waxholmsbolaget boats from Strömkajen run all the way on an SL ticket

Vaxholms Kastell is an island fortress in the Stockholm archipelago whose history goes back to the early 16th century. Its museum traces 500 years of coastal defense, with what is likely Sweden's largest collection of sea mines and a hydrophone station for listening to passing boats.

Other rooms cover spies and animals at war, with interactive code stations, and the basement recreates fortress life in the 1890s.