stillimproving, i tested a setup with two white towers, and i lost ~100 lvls. 3 different coloured towers must be the way to go... they do more total damage. But well, we'll se in some time if i'm catching up to you, might take awhile:P
i had a white tower wich i thought was pretty well placed. it was worth about a billion. anyway, i started thinking it didn't do any damage.
so i sold it. and i didn't loose one level! i put the money on my three colored towers and earned about 50 levels.
wellwell, you learn as long as you live...
is there a point in upgrading your three coloured towers equaly? after all, only one upgraded colour-tower give you more total dmg, and also a higher level = more money gain to upgrade the other two.