This is an especially delicious level, it's fiendishly difficult, which I love. I'm (re-)playing the 3rd mission on Expert difficulty, which has me attempting to break into the Shoalsgate police station (or whatever it was called), without being seen, to frame the 2nd in command. There are probably 40 watchmen in the place, since it is their headquarters after all. You can't knock any unconscious or kill any either, since that would make it obvious that someone broke in. You have to be invisible, essentially. Not an easy task. I love it. :)
How many first person shooters can you name where you have missions where a requirement is to avoid killing or even incapacitating anyone? :) The people at Looking Glass were geniuses. RIP.