Secret level 10:
To reach the secret level 10, at the end of the first level, do not go through the exit. Instead, turn right and open the secret door located in the corner of the wall. Turn right to open another secret passage that leads to an elevator that takes you to level 10.
You Are Death Incarnate - Complete one floor on "I Am Death Incarnate".
Escaped from Wolfenstein - Complete episode one on Don't Hurt Me or harder.
Operation: Eisenfaust Complete - Complete episode two on Don't Hurt Me or harder.
One Fewer Fuhrer - Complete episode three on Don't Hurt Me or harder.
A Dark Secret Revealed - Complete episode four on Don't Hurt Me or harder.
Sentenced the Madman - Complete episode five on Don't Hurt Me or harder.
Confronted - Complete episode six on Don't Hurt Me or harder.
Hey, That Wall Moves! - Find one secret.
One of Many - Exit any floor at any difficulty.
None Left Standing - Get 100% kills on any floor on Don't Hurt Me or harder.
Clean out the Castle - Get 100% of the treasure in any floor.
Blazkowicz-ed! - Complete all six episodes on Bring 'Em On! or harder.