About Tower Defense Games
Tower defense games are about spending wisely. Enemies follow a route toward your base, and you place towers along the way to stop them. Each wave is tougher than the last, so what you buy now decides whether you survive later.
Put your first towers where the path bends, so that they can fire for longer. Mix different kinds of damage, since some enemies shrug off a single type of attack. It usually pays to upgrade a few good towers rather than scatter many weak ones.