Kevin_P86 wrote:For the record, the strategy is to get the interest up to 12 (or even 15, perhaps) right away, and avoid spending money at all costs.
Given that you realize this, I'm surprised to see that your final cash total was so low
Every time you earn interest, you also earn about twice that many points (the difference appears to be due to rounding), so at least on the easier levels keeping around a huge bankroll earns you far more points than you'll get just for destroying the vectoids. The penalty for losing lives is also fairly minor (you lose as many points as you would've received for killing the enemy), so it's (probably; I haven't done the math) not worth spending cash early on if the only thing you're not going to destroy on a wave is the bonus guy (assuming that you can get it when it comes around again--those things have some serious damage reduction).
I'm also not sure when upgrades are the way to go, vs. buying another tower. I've seen notes elsewhere that suggest that a second tower is always better, and I think I agree -- all other things being equal. However, there are many potential inequalities....
(I, too, haven't finished any of the hard maps).
The times when you want more towers rather than to upgrade seem to be when the tower has side effects, such as the L1 blues (which are awesome), or when the upgraded tower would be grossly overkilling whatever it's hitting (you'd rather spread out the damage).