Short answers, pulled from the story.
Theories place Jomsborg near modern Wolin in northwestern Poland, northwest of Usedom island on lands submerged beneath the sea, or between Ruden and Greifswalder Oie. Ruins discovered in Wolin during 2023 are under investigation as a possible site for the fortress.
Knytlingasaga credits Danish king Harold Bluetooth with building the stronghold during the 960s while the Jomsvikinga Saga names Danish Viking Palnatoki as its founder instead. Historians debate whether the destruction was real or if the place never existed at all.
Heimskringla records that Dano-Norwegian king Magnus the Good destroyed the fortress in 1043. The burning of the stronghold marked the end of the Jomsvikings as an organized military force.
Harald Bluetooth died at Jomsborg in 985/86 according to inscriptional evidence. A modern memorial in Wolin bears a Danish and Polish inscription commemorating his death in 986 on the southeastern tip of an isle in northwestern Poland.
The oldest records indicate the harbor held space for three ships while later accounts suggest capacity for up to 360 ships. This tower mounted catapults on an arch spanning over the harbor entrance which could close via an iron gate.