smax tips for 258k

saving every last gold you can in the early levels is absolutely key so you get the best effect from your interest. as pyro said, the best build order for the first 3 levels is:
1: 1
2: 1+
3: 1++
keep this one tower up to air level 8. you will probably leak on 6 and you will definitely leak on 7, but you will always start level 8 with more cash than if you built 2 towers to stop the leaks.

on first boss instead of 2 fully upgraded arrows, you can beat the boss with 3 carefully placed arrow towers: 1++, 1+, and, and 1 with no upgrade at all. this saves you 26g which is HUGE in the long run with interest. you should also place the un-upgraded tower near the end, so you can sell the two more expensive towers before the round ends and save even more money.

for level 12, 2++ is fine.

3++,1 air is good enough for 2nd air with good placement, as is just 4 arrow towers for boss 2 with good placement. 3rd air i find 8++ is more than enough.

pyro's directions for the levels past 13 worked pretty good for me.

if you beat 38 just fine or with only a couple leaks, then you are done building. for 39 just buy lives as needed.

tip for stacking: if you are having trouble stacking where you want, place the white square where you want it and right-click to bring up flash settings. then move your mouse to the edge and the white square won't move. 1 left click and the tower should be placed.