Great game. My personal strategy is to build lots of riflemen. They're fast and they are inexpensive and numerous. Of course when tanks are introduced it becomes an arms race to see who can build the most tanks.
Heres a stragegy you can use if you feel like not conquering that much, or just conquering one side of enemies off of you land, if that makes any sense. Line your borders with walls and have all 25 of you garrison consist of builders.