After defeating the final boss in the dangerous tower of Zerobilon, there is a doorway that can be opened by pushing a button in the final room. Inside you will find a portal that will take you to 'Weltenraum' (Hall of Worlds).
When you enter the Hall of Worlds, open the large door by pushing the button next to it. You now arrive at a room filled with large stone spheres. One of the stone pheres has an opening with a portal that will take you to 'Steinwelt' (Stone World).
There you will stumble upon some dead Star People and their airship. You will automatically get a new quest called A Game Grotesque.
Now the Tower Defense game starts. You have to stop waves of enemies from reaching the airship. To do this, you must place towers on the platforms next to the road. To place the towers, you need to find Tower Creation Orbs, which are scattered around the plane.
Don't try to kill the enemies yourself! They are immune to your attacks and can kill you instantly! Let your towers do the dirty work.
After you've defeated the final wave, you should check out the portal from where the creatures came. There you will find the powerful blade Schneller Stahl (Faster Steel). It's not as powerful as Soul Flayer, but you can swing it extremely fast.
After this game, a new portal opens in the Weltenraum granting passage to World of a Thousand Towers. This world has an advanced level of Tower Defense. There you will automatically get the quest Advanced Tower Defense.