Re: Battle of Mohacs
I very much doubt they have had over 250 men in an unit or 20 units in a stack unless they have modified .exe file or what they are.
What I think they have done is that they have recorded many differend battles, such as the scene with massive army of horses, and cut those scenes so that the video looks connected. For the panoramic views over the battlefield they could have used video editing by cutting and pasting units here and there to archieve the sense of massive battle, but considering the camera rotations and movements it would have surely taken quite a lot of time! Also I think they have fought the battles in multiplayer mode coordinating their army movements so they look and act like a single one. 
And agreed, brilliant video!