To play, click the enemy at the middle of the screen. You earn points doing that. The red boxed sprites (heroes) are good. Hover your mouse pointer over the one you want to buy and press space. To activate the heroes, simply click them. If you try to buy a hero with less money than needed, then all of your money will be taken. You can not activate a hero if they are red. Just if you are wondering, yes, there is a boss.
Every 2 hearts I will add a new hero until the screen is full. Every 2 stars I will add a new monster. Every 12 hearts or stars combined I will add a new boss.