Rumlaborg was built in the 1360s by Albrekt of Mecklenburg to replace Jönköping Castle, which had been burned in 1309. The fortress stood where the Huskvarna River enters Lake Vättern, at a junction of waterways and roads. Its surviving remains are an irregular mound, two ramparts and three moats.

You can look around the grassy earthworks and see the pavilion on one of the mounds. Estate owner C F Mörk had it built in 1845 as part of an English-style park. The fortress was besieged several times and burned during the Engelbrekt uprising in 1434. It was repaired in the late 1440s, then fell into disuse after Gustav Vasa began building Jönköping Fortress in the mid-16th century.

Archaeological work took place during restoration in the 1930s. Finds from the site are held at Jönköping County Museum, while a smaller number are displayed in the history exhibition at Kruthuset, home to Huskvarna City Museum. West of Rumlaborg, the Huskvarna River runs past other attractions, including Husqvarna Museum, Smedbyn and Pustaleden.