Looming over town, Carlstens Fästning is a fortress built in the 1660s after the Swedish takeover of Marstrand and Bohuslän. Marstrand's ice-free port was key to trade, so King Karl X Gustav built the fortress to defend it. The port continued to be fought over for decades. Carlstens has also been a prison, known for especially brutal conditions. Admission includes a guided tour. It's worth walking up even if you don't go in: the views of the archipelago are stunning.
Inside, you can explore a secret tunnel and in the prison cells learn the story of Lasse-Maja, a cross-dressing thief and local Robin Hood figure.
On sunny days, people picnic on the lawn among the cannons. A nature trail ('naturstig') criss-crossing the island starts from here. There's a cafe at the entrance serving sandwiches, coffee, cakes and waffles.