Good game.
I found it hard at the start. But it's very good once you get the hang of it, I had it done so well, I could just leave it and nothing would get past, even on the very hard one.
Anyways, If you are having trouble getting started, this is what I did.*^
1. Build 3 machine guns
2. upgrade them all to level 2
3. build 2 more machine guns
4. upgrade all of them to level 3
5. build a flamethrower and 3 ice cannons
6. upgrade the flamethrower once and the ice cannons to level 3
7. start saving up to build lots of lightning towers.
8. Everytime you build a lightning tower upgrade it to level 2 before building another one
9. You should be sweet from here on in, build what you like.
*Where you build them doesn't matter to much, try build things where there range can hit lots of guys, with flamethrowers put it on a line, so it can hit LOTS of guys at once
^You will have to save up money, so just wait for it to pile up
It's a good game overall.