Oh man don't get me started... My friends and I got so into it we created a list of all the upgrades, their costs, their range and their damage. Yes, we felt nerdy to say the least.
I found out on my own that the cannons fire fastest when they are placed head-on with the oncoming dots. In the beginning stages I place a "nature" cannon at the end of the first turn and upgrade it, and place another nature cannon at the end of the second turn. The important thing is to fully upgrade one cannon at a time. I choose to stick to nature cannons because their final damage is greater than the other cannons and its cheaper than the slow, super expensive flower cannon(which is impossible to get fully upgraded.)Just keep on buying nature cannons and upgrading them, lining them up behind one another at the ends of long corridors.
This game wasted so much of my time it's not even funny. I got to level 84, but only after reading other people's strategies, which were somewhat like mine, just more refined.
The problem with this game is that it's unbeatable. The enemy's level increases exponentially while your money only increases linearly.
My friend found a better version of the game here:
http://novelconcepts.co.uk/FlashElementTD/
You can buy, sell, upgrade, and even increase the rate at which you gain interest on your money!