Press space to start playing, WASD to move around. I had a really hard time playing it though, crashed a lot and didn't always let me walk in every direction. Gave up after 3 minutes.
The controls are based around using dual joysticks. With a keyboard there are 8 directions of motion centered around the S key, and 8 directions of pointing your gun centered around the L key. Pressing L fires the gun.
http://www.oldskool.org/pc/BCW/BeyondCastleWolfenstein.txt