I was thinking a bit about this issue and decided to ask more informed than me. ;) The question is: Do we know anything about Jomsborg and Jomsvikings for a fact? Should we believe what sagas say about them? Supposedly they were famous mercenaries, are they mentioned in any written document as being hired by any contemporary monarch or something? Any sure dates about their beginnings? I have read what wikipedia says about them but I don't really believe wikipedia much on these semi mythical themes.
Thanks in advance and cheers from Croatia. :)

I actually had to look this up. Some background may be useful for others as well.
http://en.wikipedia.org/wiki/Jomsvikings

Within the past few days we had a post inquiry about runic engravings on a famous blade (contested as 13th century) which has some striking similarities with key names of the "heroes." http://www.myArmoury.com/talk/viewtopic.php?t=17084

There seems to be secondary or "circumstantial" evidence. In terms of surviving accounts, the situation with written evidence of settlement in Greenland/ Iceland/ North America-Canada is similar. We do have archeological evidence that the settlements actually happened.

Whitch heroes? Asleik and Amund where quite common names in medevial Norway.
The tradition of marking posessions with "X made me, Y owns me" runes continued well into the 13th century, which have been shown by various urban excevations unertaken from the 1950s onward. The scholars Oakenshott mentions made their analysis before medevial rune use was that well documented.

The blade of the Korsødegård sword is also of a distinctly later type, causing pressent day Norwegian schollars to date it to the 12th or 13th century.

As far as my limited knowledge goes,there are some things i like to mention: first, there is a reenactment group situated in the area by the name of "Jomsborg Vikings Hird"; second King Burisleif is said to be rendering of first Polish King , Bolesław Chrobry (but events in Jómsvíkinga saga suggest earlier ruler under this name, probably Prince Mieszko I, who was father of both Bolesław and Swietosława, in one interpretation associated with Sigrid Storråda). The Slav-Scandinavian link for Jomsborg is emphasised during many events, it also functions in Polish popular culture (there is a trilogy of books called "Saga o Jarlu Broniszu" that incorporates events leading to Battle of Svolder)

So basically nothing but sagas written one or two hundred years later and some slavic folklore? I certainly don't want to disregard any of these, much of the history of Croatia rests on folklore legends and some of them are very probable and more or less accepted as history, but I was hoping for something like Anglo Saxon Chronicle mentioning how this and this king hired Jomsvikings to fight off invaders... ;)

More likely, some chronicle identifying invaders as Jomsvikings, but in general, I think you're right. Remember, the chronicles were written by guys that were lucky not to meet the receiving end of an axe, they hardly have time to make proper introductions :) English chroniclers did not tell from which nation the offenders come (if i recall correctly they are described either as Danes or Normans , and I'm not entirely sure if even the word nation apply here, rather than allegiance )
Sagas regarding Jomsborg come from Iceland, which as far as i know were in Norway's sphere of influence, while Jomsborg was mostly under Danish control (and maybe Polish, from 980 -1000, big maybe). possible existence of settlement was from ~960 to 1043 , about 80 years.
As far as I know, there is no proper identification of the site (which should be distinctive if we believe the sagas, with walls, towers, inner harbour, "sea gate" etc ...). So either there's something big buried deep somewhere on Baltic coast, or thing is a legend.
We have some hard data on other settlements that were founded in same time frame in vicinity (like Kołobrzeg /german Kolberg/ with it's short-lived diocese). There are big, old settlements there too (like Wolin aka Livilni (Thietmar, 1000AD), possibly Jumne in Adam of Brema) The more strange is lack of such evidence for Jomsborg., which should have influence on local settlements if the stories are right.

[quote="Lukasz Papaj"]More likely, some chronicle identifying invaders as Jomsvikings... [quote]
Just as good as far as I'm concerned... ;)
Maybe their fortress and harbour couldn't be found if they were wooden and destroyed in 1043 by Danes?

This is a very interesting thread, gentlemen. Please continue!

Thorkjell the tall is mentioned in Aglosaxon chronicles, as he lead a major incursion there in 1010, and later stayed there.
He was, according to the sagas, a jomsviking, and brother of the Jomsviking Jarl Sigvald Strut-haraldson. I do not know if the chorniclers note this.

However, the nature or location of the jomsvikings or Jomsborg remains unclear.

One theory is that jomsborg was a kind of (semi) independent outpost. In viking age scandinavia, banishment was a quite common punishment for various crimes. As a result, somewhere in the same culture outside the jurisdiction of the various national laws would be an atractive option. Many of the Icelandic settelers, for instance, where banished men.

The baltic sea was also a good area for both trade and plunder, providing opportunities for men that owned nothing but their weapons.

The description of Jomsborg sounds pretty much as what you imagine a pirate cove to contain. In real life, it might have been a lot smaller, with less features. Keep in mind that the largest cities in northern europe at the time only had a couple of thousand inhabitants. But Hedeby, for instance, had the described features.

About the wooden features- well they sometimes preserve just good, like the hook-reinforced earthen ramparts at Gniezno (Polish capital in X century) or Poznan (X/XI c) (there are pictures of these here: http://www.giecz.pl/index.php?go=kurnatowska_2000 , pictures 2a and 2b, preserved fort/ fortified city of Grzybowo : http://www.polskieszlaki.pl/zdjecia/opisy_pun...5676.jpg).
Wolin/Wollin/Vineta was such reinforced by Mieszko I (after 21 september 967, when it was incorporated into Polish Principality ) . Data from digs suggest that Wolin at the time was biggest city in principality, having about 9000 inhabitants (capital cities, Poznan and Gniezno are estimated 4000 in the time), and one of the biggest harbours on Baltic, with 300m long quay. In folklore Jomsborg is said to be in the vicinity.

The pictures from current attempt of reconstruction (not on historic site) 1 2

Looking at a current map of region , there are plenty of places for Jomsborg to hide

It would seem much larger places than Jomsborg have eluded us for longer. I wonder if anyone has really spent that much time looking for it.