We opted to take the ferry only one way between Stockholm and Vaxholm. This was a disition based on the fact that we had already spent a lot of time on boats, seen numerous islands and wanted to save our money.
The ferry is nice, but particularly for those on a budget, it can be expensive. If you are tight on money or have a Stockholm transit pass, the bus to/from Vaxholm is free.
The best way to do it is to take the Metro to Danderyd Hospital, which is the second to last stop on the red line to Morby Centrum.
From there, transfer to bus #670, direction Vaxholm.
The overall transit time will be about 50 minutes from Stockholm Center.
From Vaxholm, take bus #670 from the waterfront near the ferry terminal. Likewise, transfer at Danderyd Hospital to the T-Bana.
You can get here by bus from Stockholm too but that would be cheating. Boat is the way to arrive in Vaxholm in style and it couldn't be simpler as it is along most commuter boat routes all year round and with a whole buch of extra departures summertime when Stockholmers go to their archipelago cottages and tourist invade the city. Then there are also the magnificent steamships leaving from Strömkajen in Stockholm's city centre. In fact, the major shipping company is called Waxholmsbolaget. Most boats have some sort of cafe and the bigger ones serve light meals and lunch offers. The steam ships have wonderful wooden dining rooms on board so a luxury idea is to catch any boat there and a steam ship back and book a table as soon as you board (or preferably earlier), or an afternoon steam ship departure where you dine before catching a bus back to Stockholm.
to come to the fortress you have to go by boat......it takes about 10 minutes....if you dont want to swim ofcourse..