Type these inside a game level and press RETURN:

wimp = God mode on/off
getalife = Unlimited (?) lives
goto = end current level
fly = fly mode on/off
pede = create a Centipede nearby
robot = automatic movement and firing until player makes a movement and fires a shot
normal = normal game speed
slow = slower game speed
sloww = even slower game speed
slowww = still slower game speed
slowwww = really slow game speed
dload = kills game and goes back to Windows desktop

The rest of the codes are probably only for program diagnostics.

diags = diagnostic codes on screen on/off
showz = screen mode (?)
boxes = boxes around objects on/off
times = time codes on/off
clearc = caches cleared
report = report files generated
purge = purge message
dumpcoords = ?
happytrails = ?
spots = ?