That's the right question for a RL German who has been in Schleswig-Holstein (the most northern part of Germany) several times – though I have not been to the very spot itself.

OK, names first
Haithabu (under this name it is known in germany today)
Heithabyr (ancient norse name)
Sliaswic/ Sleswic (Saxon name) – the close town of Schleswig still carries that name
Sliesthorp (frankish) – means village (modern German "Dorf" upon the Schlei – that's the name of the river nearby)

As to my sources, the first reference was made in the "Fränkische Reichsannalen" (forgive me for not having an English name for this at hand). There it is said that King Gotrik (or Godfred) of Denmark had come to Haithabu after having destroyed the Slavian? town of Rerik, transplanting the merchants of that city to Haithabu, thus laying the foundations of one of the richest and most thriving cities of the Viking area (the area within the wall is twice as big as Birka in Sweden). As to the last written information on Haithabu, the place was destroyed in 1066 by Slavs.