Built in 1735 to guard the sea route into Stockholm and cited as Sweden’s strongest fortress of its era, Fredriksborg’s 18th-century officers’ buildings were converted into a 15-room boutique hotel in 2013–15. Set on the northern tip of Värmdö island in the Stockholm Archipelago, the hotel commands dramatic views over the Oxdjupet strait, where cruise ships still pass daily. Guided fortress tours, sea kayaking, and cycling round out a unique archipelago stay.