If you have not played Stalingrad, I suggest you should give that a shot before your play Stalingrad 2, because this one is tougher than Stalingrad 1. I have not been able to beat the first invasion after trying for quite a while.
There are number of new additions, beginning with the need to construct factories to be able to research new weapons, and create money factories to boost your income at the end of each level. Every time the airplanes come I seem to be failing miserably so prepare air support.
The lovely new feature is that you can send troops from the end, that will help slow down or destroy incoming troops as well. Unfortunately you do not have airplanes so this is where it actually begins to get tough.
Hope you have a blast with another fine strategy game, Stalingrad 2.