• Paths only, no lawns: the house rules ask visitors to keep off the grass, so ball games and picnics on the ramparts are out
• Bikes and scooters stay below: they can use the main street, but go in the racks before you climb the rampart paths
• A working military base, so follow the signs and the guards' instructions, and keep kids off the cannons

Kastellet is a five-pointed star fortress from the 1660s, still a working military base and one of the best preserved in Northern Europe. A path runs along the top of the grassy ramparts, looking down on the moat, the long rows of old soldiers' barracks and the Citadel Church.

On the King's Bastion stands Kastelsmøllen, a windmill from 1847 and the city's last working mill.