Thief, thief, thief, thief, Thief! [http://en.wikipedia.org/wiki/Thief_(series)]
Firstly, paralleling an observation made by Yahtzee (in his reviews of Splinter Cell and Velvet Assassin) most of the levels are non-linear, so you can choose your approach, either to clear one perimeter at a time, or to sneak directly to your objectives and worry about looting later. Many of the levels also provide secret passages and transit ways between zones that are often unknown to the guards, and provide for hasty escapes. Some levels in Splinter Cell: Chaos Theory provide a range of different approaches, at least within a zone, with no right or wrong way to go about it, especially when figuring out how to get close enough to a hostile to make a clean neutralization[footnote]Dropping down on a target from above, or defenestrating [http://en.wikipedia.org/wiki/Defenestration] someone from outside the window are two personal faves.[/footnote]
Secondly, I dug the code-against-killing that Garret takes on when playing Expert difficulty, which positions thieves and spies (or saboteurs) in a higher order than warriors and assassins.[footnote]Interestingly, this has marked how I play the Splinter Cell series, in that I tend to get zero kills and lots of knockouts, mostly from Uncle No One Lives Forever: A Spy In HARM's Way [http://en.wikipedia.org/wiki/Sam_Fisher_(Splinter_Cell)] in which a knocked-out foe would waken on his own and eventually arm himself, even if all weapons were confiscated. So the best way to dispatch foes was to tranq them from afar and then shoot them in the head with a silenced handgun. Excellent spy grit, but dissonant with the light-hearted themes of the franchise. In Splinter Cell, unconscious bodies can be wakened if discovered by allies, which some players took as incentive to just kill them in the first place. Unconscious foes in Thief stayed that way. So, a note to game devs: knock-outs, like flashlight batteries, should last the duration of a level.[/footnote]
One of the problems that I've seen in Thief, but also in other games, including the Splinter Cell franchise is the level of blue sense [http://www.skepdic.com/bluesens.html] that searching enemies show. They seem conspicuously drawn in your direction, no matter how hidden you are, and in the case of Splinter Cell no matter if you've tossed a sound distraction somewhere else in the room[footnote]The noisemaker arrow in Thief seems to just as magically distract guards, so despite their prior intuitions, the noisemaker is far more attractive a target[/footnote]. I've also noticed that in a lot of cases, if one person detects you enough to shoot at you, everyone else in the area immediately knows exactly where you are (enough to shoot you a lot), no matter how dark it is. Sadly Thief: Deadly Shadows took things in the opposite direction, making opponents really easy to confuse, and having the wall-hug maneuver make Garret completely undetectable to everything else.[footnote]Ergo, a common solution to a disturbance would be to wall-hug and then go make a sandwich. By the time you got back, everyone would be back to their patrols.[/footnote] It seems to be an ongoing challenge to make enemy AI believably smart and believably stupid.
As a last note, Level 3 of Thief: The Dark Project, titled Down in the Bonehoard is in my opinion, the best dungeon romp ever! One could (and someone should) devote an entire game to sneaking into tombs of the rich and powerful guarded by traps and ill-perceptive (or plain stupid) monsters that completely outmatch the looter in a stand-up fight, to relieve the dead of treasures they are obviously no longer using. The story writes itself: Tomb Robber makes a meager living on commissions to reappropriate family heirlooms and legacy weapons. Said delver earns a fixed fee plus tips, that is, any treasures found not specified by the client. Sooner or later, our catacomb crusader will piss off some god or undead king or whatever by stealing the wrong thing or unintentionally desecrating some holy spot, and the game is afoot.
238U