Isegran is a small island where the Glomma empties into the Oslofjord, connected to Gamlebyen by a short bridge. The island has been fortified since medieval times — a tower here guarded the river mouth long before the 17th-century fortress town was built — and the ruins of the old fortifications can still be seen among the grass and rock.
Today Isegran is home to the Østfold Boat Museum (Østfoldmuséet Isegran), which holds a collection of traditional wooden boats from the region, and a boatbuilding workshop where craftspeople still build and restore traditional wooden craft. The museum is modest in scale but the island itself is worth visiting for the outlook over the river mouth and fjord, and for a sense of the strategic position that made this spot important for so many centuries.