I agree with the bank upgrade. You could also possibly add higher interest rates for higher balances. Also, maybe add an overall stats page somewhere, with all the obscure stats collected in one place.
Interesting discovery. If you find yourself at the end of the level, and one of the creeps has wandered off the map somewhere, sell everything, then build a sniper tower and surround it with range boosters. I had a sniper tower with a range of over 21,000 that picked off the missing creep.
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