The Fredrikstad Museum is housed inside Gamlebyen in a building that was once a military storehouse. Its permanent collection covers the history of the fortress and the city: how the fortifications were built, who lived inside them, and how the garrison was organised. Scale models of the bastions make the engineering logic of the star-shaped layout immediately comprehensible in a way that walking the ramparts alone does not.
There are also exhibits on the town's industrial history — Fredrikstad grew in the 19th and early 20th centuries as a significant timber processing and shipping centre, and the museum holds artefacts and photographs from that era. The museum is a good first stop before wandering the streets, giving context to the buildings and layout you will encounter on your walk.