Stretching north of Gothenburg towards Norway, the name Bohuslän is derived from the Bohus fortress, 25km inland. Yet once this stretch of coast was acquired by Sweden in 1658, it was Marstrand’s largely ice-free harbour that commanded the sea and still pleases the eye. The castle was begun soon after and quickly forged its own self-serving punishment. In 18th- and 19th-century Sweden, “Marstrand labour” was just another term for working on the fortress with an iron ball around one’s ankle.

Hundreds of shackled criminals built the walls, paved the courtyards and raised the great round keep. There were a few escapees, notably the Robin Hood-esque Lasse-Maja, who dressed as a woman to entice and rob wealthy farmers. Marstrand became a free port and was among the first to get electricity. The return of great herring shoals meant the town prospered, and by the 19th-century the upper classes decided this was the place for curative baths and holidays.