So we're on Rügen; the beatiful island on the coast of Germany. This used to be a part of DDR during the cold war. Of course there are still many things left from this period. One such thing is the Dwasieden castle. There is no english wikipedia page for it, unfortunately, but at least a short german one: Dwasieden on german wikipedia
In short, the castle was used by the Kriegsmarine during WWII. After the war, the Red army destroyed most of the castle but used the area for military purposes. There are industrial buildings (more or less ruins) left in the area, what seems to be bunker entrances and many remnants from military and industrial activitivies. I would love to know more about it, but it seems hard to find info in english.
Anyway, most of the buildings and roads in the area are quite well mapped, so when I arrived it seemed there was not much for me to add. However, the actual castle did not have any outline in OSM, just a single node. I did not have much time for my visit, but I traced one lap around the ruins, and walked a bit on what was left of the second floor in the southwest wing.
When I got home I used the blueprint found on Wikimedia commons and tried to adjust it to my gps track, using JOSM. Unfortunately my track was not very accurate, but I tried anyway. Since the blueprint had a scale ruler on it, I could at least adjust the scale of the image fairly accurately in JOSM.
Probably someone may improve this work, but it was a very interesting visit. One could spend days there mapping things.