Allegedly guarded by a giant and the final resting place of the legendary Viking king, Harald Bluetooth, the truth about this Viking stronghold is a mix of historical fact and mythical fantasy, captivating imaginations since the early medieval period. 

Deeds of the Bishop of Hamburg 

Trying to sift through the sands of time to pinpoint the location of a semi-mythical Viking stronghold – mentioned in numerous sagas – could send a modern historian on a wild goose chase. 

Like so much of early medieval Nordic history, we owe profound thanks to the renowned medieval historian and scholar Adam of Bremen.

In his seminal treatise, Gesta Hammaburgensis ecclesiae pontificum (Deeds of the Bishop of Hamburg), Adam of Bremen chronicles the history of the Viking Age

Beginning with the foundation of the Hamburg bishopric in 788, which was tasked with a mission to convert the pagans in Viking societies scattered throughout Scandinavia, he traces events right up to his contemporary time in the mid-11th century. 

The treatise has proved an invaluable source for the study of the Nordic region during the early medieval period, with everything discussed and analyzed, from the state of paganism in Scandinavia with the coming of Christianity to the "discovery" of a new coastal region far west of Greenland. 

However, it is in Book 4, where Bremen provides a geographic overview of Northern Europe, that he saves the best for (almost) last. 

Here, he describes a Viking stronghold located somewhere in the Baltic Sea, home to the Jomsvikings, fearless warriors. 

Enclosed within a fortress with nearly impregnable high walls, Bremen asserts that the elite warriors here adhered to their own warrior code, a sort of Viking "bushido." 

In many respects, Jomsborg and its Jomsvikings can be seen as an early medieval precursor to the renowned Al-Alamut castle stronghold, the fortress of the notorious Nazari assassins, later popularized by Marco Polo's tales. 

However, how much of what Adam of Bremen recorded is factual? Did a Viking stronghold truly exist in the Baltic Sea? 

Based on recent archaeological excavations, Wolin Island in northwestern Poland has been proposed as a possible location of the famed Jomsborg fortress. Photo: Janusz-Gajewicz / Shutterstock

A budding Indiana Jones 

Ten-year-olds rarely make the news – unless there's a touch of tragedy or genius – and even more seldom do they make historical and archaeological headlines. 

However, in 2014, when 11-year-old Maja Sielska brought a box full of old buttons to show her teacher in Wiejkowo, Poland, she did more than just make headlines. 

The box had belonged to her great-grandfather, Major Stefan Sileski, who had raided an ancient cellar crypt towards the end of the Second World War in 1945. 

Among the Viking-era treasures discovered in the crypt was a disc. Sileski had placed it in the box, dismissing it as worthless because it wasn't made of gold. 

The disc remained in the button box until 2014, when Maja, Stefan's great-granddaughter, received it as a gift from her family. 

Whilst working on her homework about the early medieval period, young Maja recalled the unusual-looking button in the box and decided to share it with her classmates that year. 

However, what her great-grandfather had dismissed as being of little value was, in fact, priceless. 

The supposed button was actually a golden disc, today known as The Curmsun Disc, weighing only 25 grams (0.8 ounces), produced sometime in the 10th century.

What was intriguing about this small disc was what was written on the coin, believed to be a transliteration of spoken Old Norse in Latin letters.

Scholars have deciphered the inscription to read, "Harald Gormson, king of Danes, Scania, Jomsborg, and Aldinburg." 

Now, Harald Gormson is, of course, better known by the moniker Harald Bluetooth, a fierce Viking king of both Denmark and Norway who ruled in the late 10th century. 

The mention of Jomsborg confirms that this was a real place – but there is still an ongoing debate about whether there was an impregnable Viking fortress in the Baltic Sea. 

Maja's discovery in the village of Wiejkowo, located in northwestern Poland, was in an area with a rich Viking history. 

Before the end of the Second World War, this part of Poland was known as Pomerania, and during the early medieval period, it was called Wendland. 

It is in Wendland that Adam of Bremen mentions the location of the Jomsborg fortress, whilst modern scholars suggest it might be somewhere along the banks of the Oder River. 

The most recent archaeological evidence discovered is the remains of a vast fortification on Wolin Island in northwestern Poland, which aligns with many descriptions in Viking sagas and historical chronicles. 

The Curmsun Disc has a Latin engraving on it, which references the Viking King Harald Bluetooth, addressing him as the ruler of Danes, Scania, and the legendary Viking stronghold Jomsborg. Tomasz Sielski / Wikimedia Commons (CC BY-SA 3.0)

A fiery end? 

The Jomsvikings were said to be known for their exceptional martial skill and loyalty. 

In fact, their strict warrior code was said to include never fleeing from the battlefield, never revealing their true identities, and avenging the death of a fallen comrade. 

The fortress served not only as a training camp to refine their skills but also had a strategic location – near the Baltic Sea, which was practically a pond for the Vikings during the early medieval period. 

This positioning allowed them to swiftly conduct daring raids. 

Legend has it that their fortress gate was guarded by a solitary giant named Mimir. 

So, what then of the elite Viking warriors who were said to be some of the most skilled and fearsome warriors of the entire Viking Age? 

The answer can be found in literature. 

The Jomsvikings gained such a reputation – whether much of it was based on truth or not hardly seemed to matter to a medieval audience – that they became the protagonists of their own saga, the Jomsvikingsaga

This saga, written down in the 13th century in Iceland, not only chronicles the adventures of the Vikings and their fortress but also intertwines certain historical figures, including Harald Hardrada

It's another saga, the Heimskringla, that provides a potential conclusion to the story of Jomsborg and many of the Vikings who resided there. 

In 1043, right at the twilight of the Viking Age, Magnus the Good, King of Norway and Denmark, is said to have sailed to the fortress, besieged it, and razed the entire stronghold, resulting in the demise of many Jomsvikings. 

This event was likely the final straw after these Vikings had often been used in the palace intrigues of the past half century, often supporting rival claimants to the thrones of Denmark, Norway, and Sweden. 

BBC History Extra has written more on the Jomsvikings, available to read here

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