On unlimited- fill up the map with towers (I use mostly 2's) then quickly upgrade some at the front while at the same time slowly upgrading all the other towers at an even pace.

If you try upgrading towers all the way before adding more you'll get to a point where you can't keep up with the mobs.

I like mostly 2's with a few 1's and a couple of 5's for the end & armored flying mobs

I upgrade damage all the way first, then upgrade speed fully

Is it just me or does this game become impossible after a certain point. Not getting enough cash to upgrade towers enough to beat down the tougher vehicles.