Already in 1548 King Gustav Vasa ordered to build a fortress at the islet. A village started to grow across the strait which in 1647 became a town. The fortress, as we see it today was completed in 1863.
Up until 1944, the fortress was used by the military. It is well maintained, and well worth a visit, as is the splendid Defence Museum, Showing how the archipelago´s coastal defence system has developed over the last 450 years. A short trip by boat from Vaxholm quayside will get you there.
Opening hours http://vaxholmsfastning.se/
You reach the fortress by a ferry from Gate no 9 opposite the fortress http://www.trafikverket.se/farjerederiet/farjeleder/Farjeleder-i-Stockholm213/kastelletleden/