I don't know your exact situation, but with only a 2 hexes wide corridor, in theory it will be hard to stop the Germans from isolating the City during the winter turns. That said, to reduce it, take it and hold it after might be another proposition altogether for them. You'll be the judge as to what forces to leave in the city, but if you decide to leave a strong garrison, make sure you can supply them when the pincers close, and keep strong mobile reserves to the east or on his flanks, again depending on the situation, to counterattack as soon as possible when blizzard hits. I think you should be able to relieve it or at least take it back if it falls. Don't forget, during blizzard, minor and major rivers become like clear hexes, no MP penalties to cross.
"The bugle sounds as the charge begins
But on this battlefield no one wins!"