Fantastic Game. Love to see more like this in the future, Or perhaps a few cheats, such as unlimited commander ammo, or unlimited money, something for when you don't want a challange, but want to shoot the guns x
Lower levels, take your time not killing the bases, every level up is skill points, so if you can get more level up's earlier, you have a far greater chance.
For levels without Nat gas, BUILD farms and max a recycling center. this should give you the gas to power vehicles and use the occasional flamethrower. my best tip for challanges ^.^
Jump in the air, then press sleep. do this 3 or 4 times and creeps can't get you. then jump and hold down to get back down for the trucks. EPIC GLITCH!
2/5, yes it's a copy, but the only new feature is bosses and spells, which frankly aren't that fun. Most TD have some unique feature to them, but those that don't do suck.