With 7 enemy types, over 11 unique tower paths, tower merging allowing for mega towers and mega-mega towers (the size of 16 towers combined!), and 52 whopping game modes, Bubble Tanks Tower Defense will keep you busy for a very long time.
Full tutorial and instructions are in game. Can be played using only the mouse, but hotkeys are suggested for experienced players. Hotkeys can be changed via the Settings menu.