RIDICULOUSLY addictive flash game challenge

[Crandaeolon]

2nd try, 10,224,076 points, 2,9M in bank, 20 lives.

3rd try, 12,136,003 points, 3,9M in bank, 14 lives (tried something a bit different.) Gonna give it a rest now for a while.

How do you get so much money in the bank? I can barely get over 2000 without having to spend it. Strategies, please?

Since it's an optimization game, it's kind of hard to offer good tips without spoiling it entirely. Minimal spoiler follows:

Spoiler Alert, click show to read: 

The general idea is to get by with as minimal defenses as possible. You'll also want to be a good investor and maximise your interest.