The system of challenges and scoring is relentless. If you are spotted, you lose points and take you to target enemies (and they do the damage!)
If you kill other characters that designated target, same ... you lose points.
Knowing that we can very quickly fall to a negative rating and that artificial intelligence can prove paranoid, you understand that it takes several tries for a correct score ... and many attempts for a respectable score. Knowing that there are various ways to achieve its goals: to dress up, create diversions, cause accidents, quietly strangle his target, execute the function for sniping, etc. And again, the most original methods are the most rewarding ... but also the most difficult to implement.
In short, a simply exceptional game, with a challenge worthy of the name.