Easier way to win...
2 cannons, upgrade in middle. When you get fire cannons, sell and put up a few in the middle.
Put the next point you get into interest, and keep adding fire towers in the middle as needed, leaving yourself money for interest.
Next point into water - put some up right away near the towers, they effectively double the damage by slowing the creatures.
Last point into earth (useless towers), but gives you rockets. You should have 1500-2500 gold, put up four rocket towers and upgrade (about 1k), and watch ultimate destruction (2000 damage huge range fast). Keep rocket towers central and put up some water towers (cheap) to slow down movers.