How to find the secret level of Wolfenstein 3D
Reference to The Elder Scrolls 5: Skyrim
During the prologue you can find a dragonborn helmet by The Elder Scrolls V: Skyrim near a box warehouse.
Below you can see the exact place.
Nightmare levels and how to complete them
This video shows all nightmare levels and how to complete them.
Collectibles and nightmare levels
Where are all the collectibles and nightmare levels.