When I was eleven years old, it was a very good year, and I can remember my daily routine vividly. These were the years before I owned a Sony Playstation, and I used to venture to my friends house - everyday after school - to watch him play through Final...
Remember to make the lizards take the longest route you can when you put down your towers.
You can see the routes flying units take, so make sure they're covered.
Sometimes, it's worth upgrading old towers instead of buying new ones.