Here is the unoffical list of levels
(notice, this might change a bit in the fullversion):
1. Grauwald,
2. Gold mines,
3. Checkpoint Alpha, (Boss 01)
4. Old entrance,
5. Office wing,
6. Dark corridors,
7. Dungeons,
8. Upper prison floor,
9. Grauwald, (Boss 02)
10. Underground passage,
11. Laboratory entrance,
12. Test area,
13. Medical wing,
14. Hidden vault, (Boss 03)
15. Hallways of Sturmgard,
16. Ramparts
17. Tore über Himmel
18. Himmlers Valhalla (Boss 04)
19. Offenbach (Bonus lev. 01)
20. Abandoned dungeon (Bonus lev. 02)
21. Fragment of Memories (Final boss)
