There's a really easy trick to beating this game...
Get the first 3 bombs and mana regeneration as first things, then increase your pop cap.
Doesn't matter what lane you put your guys in, really. Place as many as you can at the very top (or bottom), try to avoid gold dudes and exploding dudes. Use your bombs to clear any initial dudes that are in the path of your giant dense mass. Use their power if you're using the lancemen/swordsmen to bumrush the line, or else use the mage's power when guys start showing up. Level up attack when you can, push when it's not available.
Spreading your guys out causes them to lose pushing power, and usually your first batch will kill the enemy (send more otherwise)
How to put the game on "fast" without having to hold space the whole level:
Make sure your internet browser is not completely full-screened (if you only have one monitor). Hold space, click outside of browser.
I don't know if this is just me, but I have more revenue than expenses, yet I keep falling into the red. Also, I lose population (and cities get uncaptured, I get that), but when I recapture the city, my population is a lot lower than it once was. Am I subject to some bug or am I just a horrible zombie conqueror?
Just a little complaint about the combo upgrade, the fact that the amount of combo needed to get fever increases (by a lot) every time you increase combo time makes it useless. Don't bother getting it.
People who are saying they would like to assign hotkeys to groups, you can!
1) To select multiple units: Hold shift and click each unit you want in the group.
2) To bind units to a group: Hold down control and press a number
3) To select group: Hold down shift and press the number.
Click the + so people know!
I think the problem is with the cost of expanding sideways. If you keep building up, the elevators will become exponentially more cluttered. A solution could be adding more carts to a certain elevator line (for a hefty sum, of course). Either that or reduce the scaling for width expansion. Else wise, this game reminded me so much of Sim Tower the nostalgia was pouring out of me.