Krudttårnet (Powder Tower) is a squat octagonal building of red brick and grey stone that dates from 1686, when Frederikshavn — then called Fladstrand — was a fortified naval anchorage. Inside its thick walls, the rooms now hold a small collection of period cannon, muskets, uniforms, and ship models, much of it linked to Peter Tordenskiold, the Danish-Norwegian naval hero who used Fladstrand as a base during the Great Northern War.
The tower is open in summer and the visit takes about half an hour. It sits in a small green at Strandvej, close to the railway station, and is signposted from the harbour. Free or near-free entry; chiefly worth a stop for the building itself and for what it tells you about the maritime history of the town.