Halden (hälˈdən) [key], town (1995 pop. 25,951), Østfold co., SE Norway, a port on the Iddefjord (an arm of the Skagerrak), near the Swedish border. Manufactures include forest products, footwear, and textiles. The first atomic reactor plant in Scandinavia was built there to furnish power for industry. The town was chartered in 1665 and developed around Fredrikssten Fortress, which three times (1716, 1718, 1814) withstood Swedish attacks. Charles XII fell before its walls in 1718. From 1665 to 1928 Halden was known as Frederikshald or Fredrikshald.