You don't even need upgrades.
The idea behind this is great, and I can imagine it being hard to do, but this game takes a good shot at it. However, this game suffers from what a lot of other games of this genre suffer from, the chees-o.
First stage, buy 10 archers, 20 infantry.
second stage, buy 20 archers, 40 infantry.
third stage, buy 30 archers, 60 infantry.
next stage, +10 archers, +20 infantry.
Send every single one of them at once (all your archers first then all your infantry). It'll lag. especially when you have like, 180 guys out on the field, but you'll win everytime (well, I have so far lol)
Keep up the good work, hope to see more of this.