I suggest normal difficulty first, if you find that too hard after a few goes trying out different strategies maybe drop back to easy.
Rich mode is just for mucking around at the start to build a tower however you want. Only play this after you've played the game properly.
- Flying won't attack your heart if it's not in line with them, but will pose a threat if you build high. So be careful.
- Cannons work better the higher they are. There range & accuracy is based on there height as you will see.
- Try to use a bit of everything, don't stick to just defensive, or just offensive.