Go to Google maps, turn on satellite, type in these possible latitudes and longitudes and see for yourself: 54.602885,20.199287 54.601558,20.205241 54.569851,20.216174 <-- My favorite one. It has a certain feel to it that makes it look like the castle.
It is also possible that the city of "Svelty" was once Alstadt, having the castle due north, however Google does not allow you to scroll in close enough to explore:
54.683061,20.133476
And yes, I know Amnesia is just a game; but the developers got things so historically accurate you can't help but wonder if this castle is still standing somewhere.