Its a different take on a FPS. You must use stealth and shadows. Weapons are a last resort and usually only good to help you escape. Each level can be solved in a variety of ways so no 1 patch is correct although you get 'bonus' points for certain tasks in a mission (i.e. complete it without being seen even once).
I played this when it first came out back in the ninties, then again a few years later. I have not played it through the gamefly client, but this is an EXCELLENT game. absolutely a classic, any serious gamer should give this title at least one playthrough.