The huge building sat rotting away over on the Old Town edge of the shipyard is Hala U-Bootow, or U-Boat Hall. It was here that the Danziger Werft company built U-Boats for the German navy and it was this which more than anything drew the attention of Allied bombers which so seriously damaged the city of Gdansk. During its operational life U-Boats would have passed through it onto a second building which would have stood between it and the slip and then from there into the river. The fact that the building is still standing today and the other part is not, is that it was so well-built to withstand destruction.
Looking at it from a distance (a good spot is the viewing terrace on top of the nearby European Solidarity Centre building) you'll see it is in fact a huge re-enforced concrete bunker. The Germans did not want their U-Boat production to be noticed from the skies, so they built this huge bunker to keep all construction hidden from prying eyes and then disguised the building with red brick to make it look like just another of the shipyards many large red-brick buildings. It would have been demolished long ago if someone had been able to come up with a way to safely demolish it without taking half the city with it. Fortunately, the city of Gdańsk knows the value of adaptive reuse, and the U-Boat Hall is now Montownia - hosting apartments, a community space, and an awesome food hall!