There was a complete history of Marienburg printed in a WD article (the very first on Marienburg) and I assume that it will be reprinted in some form in the sourcebook. Until then, here is a summary of the major historical events in Marienburg's history, taken from the above sources and an Internet text by Anthony Ragan:

-4550 to -2000: elves founded Sith Rionnasc'namishathir, "Fortress of the Star Gem on the Sandy Coast." They were joined by the dwarves who provided much construction work in the city.

-1997: to -1500 The Dwarf Elf wars that split the old world were the cause of the cities downfall. Although the dwarves won control of the city after a protracted siege, they left after several decades, raising the city to the ground.

The date of human arrival is uncertain. (Circa 0 to 100 IC): A tribe of humans called the Jutones were operated in this area of the Old World, and it is likely that these were the source of Marienburg's founder. The Jutones were fiercely independent, being the last tribe to swear allegiance to Sigmar Heldenhammer. As the Empire established firmer control over its territory, the title of Baron of the Westerland was granted to Marieus, a Jutone descendent. Marieus founded Marienburg in his name.

765 IC: treaty of Traktatsey. Concluded at Althing, it ended the long wars between the Norse and the Empire, and inspired trade. Of interest to Historians, as is the earliest original documentation of Marienburg still present in the Old World today (in the University Library).

936 IC: Marriage with Duke of Mousillon

1087 IC: Treaty of Pend seals the relationship with eastern Albion. There have also been several, shorter lived treaties in the preceding years with other countries, but the overall effect has been to increase Marienburg's influence as an accessible trading port.

1152 IC: Marienburg, being removed from the centre of the Empire is unaffected by the 200 year Age of Wars.

1597 IC: Marienburg is seized by the Breton Army, but after five years the Empire forces their retreat.

2150 IC: Marks the important date when the Sea Elves returned to the Old World, and Marienburg offered their original Land back in exchange for exclusive trading rights. Marienburg now plays host to the largest Sea Elf Enclave, with all the riches that entails.

2302 IC: The last of the Van der Maacht Lineage and Baron of the Westerland was killed in battle in the last incursions of Chaos. He was 15 and left no heirs. A power squabble immediately broke out amongst the other Imperial Nobility, all of whom tried to claim that they had rights to Westerland. But three powerful Marienburg merchants, Jan Koopmans, Pieter Winkler and Thijs van Onderzoeker approached Magnus (Emperor) and proposed that a council of merchants run Marienburg to save political favouritism.

2305 IC: the edict was signed that officially changed Westerland from a barony to a province, still remaining part of the Empire.

2378 IC: Successful campaign against raiding pirates by private Marienburg's Merchant Navy allows the council to persuade the Emperor to disband the Imperial Second Fleet in Marienburg. Marienburg also appointed its own excise officers, with the Imperial Tax men merely checking the books. The true power had shifted to the side of the Merchants.

2429 IC: The Wasteland formally seceded from the Empire, due to punitive Imperial Wars and Taxes. Prince Wilhelm attempts to regain the city three times, but a war Exhausted Army fails.

2499: Marienburg Present Day

Present Day Relations Between The Empire and Marienburg

Looking at the history, you can see that Marienburg has only been independent from the Empire for seventy years. Although the majority of tension between the two has died down - Marienburg is far too important to the Empire's trade routes for it to be ostracised for long - Marienburgers will be treated slightly coldly if they choose to visit the Empire.