Close-sounding names are tourist-confusers. When confounded, make a little list.
1. Fredrikstad is one fortress, south of Oslo, and close to sea level. 1567. A walled town, see http://walledtowns.com/towns/fredrikstad/
2. Fredriksten is another fortress, also south of Oslo, but farther south than Fredrikstad, more inland, on an estuary-river area, on a high promontory. 1661.
3. Fredrikshald. This is the town of Halden, at the location of Fredriksten. http://www.theodora.com/encyclopedia/f/fredrikshald.html. At one point, the Swedes did take the town of Halden, but the citizens burned their own houses, preventing access to the hilltop fortress.
Other town name-alikes: Kristiansand, northwest coast, in Nordmore; and Kristiansund, south, at Vest-Agder.
The Fredrikstad-sten-hald group were all needed because Norwegians had ceded to Sweden in 1658 a huge fortress built by Norway at Bohus in 1308, still farther south. Bohus is still part of Sweden, see http://swedenroadways.blogspot.com/2011/06/kungalv-bohus-fortress-castle-prison.html, but there had been a series of complex interactions, war and peace. Norwegians thereafter feared invasion as alliances shifted. Thus, these new fortresses in the 17th Century. Neither retains any military significance now. The old gun emplacements remain, however. Only a lower fort area of Fredriksten was taken, briefly.
1. Who is behind the names? Fredrik II and Fredrik III, and spellings Frederick. And Frederick1. Broad reach of centuries, areas, same names used. Also confusing. This Fredriksten is associated apparently with Fredrik III, see http://www.spottinghistory.com/view/3184/fredriksten-fortress/. Distinguish? Quiz in the morning.
The von Mansbach monument was relocated here from the town. See Map of Fredriksten at http://www.forsvarsbygg.no/ftp/verneplaner/Fredriksten_eng%20HQ%20visning.pdf/ He had purchased vast tracts of adjoining land, now part of a Foundation that contributes greatly to the support of the Fortress through donations.