How to beat level 2
Start by placing turrets by the side of the road, far to the right, just where the enemies come in. Build more turrets until you don't have the energy, and whenever you get enough money build a power plant until you have 5 power plants. As soon as you have 5 power plants, you should have maybe 14 turrets, 7 or so on each side of the road (that's a guess, it could be more or less). Then wait for 1000 cash and 100 energy. While you're waiting, when any of your turrets die, replace them immediately. As soon as you have the stuff, get a factory and buy the upgrade allowance. Then (still replacing your turrets if they die) start clicking on turrets and clicking the upgrade turret button at the lower left of the screen. The tanks that come in should start dying immediately once you've done some upgrades. Get 3 or 4 upgrades per turret, and don't bother to upgrade them unless their health is fairly high.