Fredriksten was built after 1665 to make sure Sweden could never again take this stretch of border, and it did its job: sieges in 1716 and 1718 both failed to take it, giving Halden its old motto, "Never conquered." The star-shaped ramparts and bastions climb a rocky hill directly above the town, with commanding views over the fjord, the rooftops below, and the wooded hills running into Sweden.
The grounds are open and free to walk at any time, with restored barracks, a commandant's residence, and cannon still in position along the walls. A small museum inside covers the fortress's military history and the peculiar death of Charles XII in more detail. Come at golden hour if you can — the view over Halden from the ramparts is the best reason to make the climb.