Unlock all levels
At the name entry, input !LAMAUDITE! this will unlock all maps.

The thing about this cheat is, that "LA MAUDITE" is the female french word for something similiar to "the damned", or "the devil", or "the horrible". The programmers implemented this cheat for one of their female bosses, who they hated enough to put a super special hidden cheat in the game for her. There used to be another cheat called !LACHIENNE!, but this one literally means "the bitch", and was a little to rude, so they chose for
!LAMAUDITE! instead.

Hiding
You cannot be seen when hiding in dark places. Shoot the lights to make it dark, but make sure that there are no guards around. Do not stay in the light too long.

Escaping a room full of soldiers
Take a sticky camera or smoke grenades and throw it near where the most guards are located. For the sticky camera, a control bar that tells you how to make a noise or use smoke will appear at the bottom of the screen. Throw the smoke grenade and the smoke should knock out some guards. When the smoke appears from the grenade(s), it is your chance to run for it .

Knock out an enemy
To knock out an enemy without killing him, equip the sticky camera and shoot it at his head. Aim slightly above the head to compensate for gravity. After he is down, pick up the sticky camera and use it again.

Defense Ministry: Getting to the South Hall/Wing
After interrogating the driver and heading back upstairs, save at the checkpoint and view the intermission sequence of the window in the courtyard opening. Walk out on the balcony and turn left. Jump on the rain gutter on the wall. Move along the gutter towards the open window. If an alarm goes off, you have not properly hidden your dead bodies. Move back through the level and throw all your dead bodies into dark corners.

Defense Ministry: Nikoladze's Office
Once you rappel through the glass window and kill the guard, access the computer in the far right corner of the level first. Then, access the main (Nikoladze's) computer closest to the window. Immediately run back to the window and drop out, but stay near to the ledge so that you will grab hold of it and hang out with the window. Guards will enter and search the office. They will not see you if you remain hanging out the window. They will clear the room, then leave. You can now access the computer again (as instructed by HQ). Then, proceed with caution when reaching your extraction point.

Luring your enemy
To do this, find a can or glass bottle to pick up. Once obtained, move to a location, preferably dark. While hidden, throw the can or bottle to an open area with an enemy nearby. When thrown, the enemy will wonder what happened and walk towards that location. After you have drawn him out, you can take him out for an easy kill