As i see some comments about the guard always seeing the light:
An essential part of the game is the toggling of the flashlight. Press L to toggle it off and on. Careful, don't do it too near to a guard, he might hear you.
There will be several parts of the game where you will have to memorize the layout before (when using the flashlight or the editor :) ) and run past a guard in the dark. Here be careful again, as when you run against a obstacle the crash sound may be heard as well.
The first two levels are piece of cake. In the third there's a bigger room with two guards walking vertically. You'll have to run up to the bathroom behind the guard in darkness. This may take several attempts.
In the fourth turn off the flashlight at once. In the last room there are two guards. Although it looks easier to walk down the left side, it is impossible. The guard will notice you when you want to walk by him. Try walking down the right side.
5th and 6th are not that hard, plenty of room to maneuver.
7th is annoying since there are a lot of cubicles to search, but can be done as well.
8th has a real bitchy room on the bottom. Be sure to have your flashlight turned off here.
When doing the 9th i ran out of time. Pretty straight forward: down, right, down, right. Flashlight off and you have to wait sometimes in a little corner to let a guard go by.
Final level, not too hard either. If you get to the room with a choice to open left and right, go left first. You'll need the keycards.
If you're still reading this, you'll get the fine part: If you want to see the other levels, type "georg" during playing to advance a level. You can also type "willi" to see the end screen (not that impressing).
Ok, now go and have fun with it!
Georg "Endurion" Rottensteiner