This is not a typical Tower Defense game where you build towers along the path. You're standing on top of a tower and defend it from enemies. Probably that's why the game is called this way. You're a dark mag who's against everyone else in the world. You need to stand waves of enemies killing them with your spells. Select the spell at the right hand part of the screen and fire it clicking the mouse. When you kill an enemy, roll your mouse cursor over the dead body to collect the soul. The souls work like money. You can buy upgrades with them or you can recharge your mana spending souls. You'll also earn experience and will be able to learn new skills or improve the ones you have in the end of each level.