Secret Level 10 - Go to the end of the first level. Instead of going through the exit, turn right, and open a secret door in the corner of the wall. Then, turn 90 degrees to your right to open another secret passage. At the end, you will find the elevator to Level 10.

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Escaped from Wolfenstein (15 points): Complete episode one on Don't Hurt Me or harder.
Operation: Eisenfaust Complete (15 points): Complete episode two on Don't Hurt Me or harder.
One Fewer Fuhrer (15 points): Complete episode three on Don't Hurt Me or harder.
A Dark Secret Revealed (15 points): Complete episode four on Don't Hurt Me or harder.
Sentenced the Madman (15 points): Complete episode five on Don't Hurt Me or harder.
Confronted (15 points): Complete episode six on Don't Hurt Me or harder.
None Left Standing (20 points): Get 100% kills on any floor on Don't Hurt Me or harder.
Clean out the Castle (20 points): Get 100% of the treasure in any floor.
One of Many (10 points): Exit any floor at any difficulty.
Hey, That Wall Moves! (10 points): Find one secret.
You Are Death Incarnate (10 points): Complete one floor on "I Am Death Incarnate".
Blazkowicz-ed! (40 points): Complete all six episodes on Bring 'Em On! or harder.