A large part of Thief can be summed up in one phrase - "stay silent, and stay out of sight". Light, shadow and line of sight each play a critical role in ensuring that you stay hidden, whilst on-screen and audio warnings alert you to dangers you could otherwise be unaware of. Although combat is possible it's not the games strong point.
A light meter in the left hand corner determines how much light you are in and thus how visible you are. If it's fully white you'll be spotted almost immediately, but if shadowed you can be extremely close and remain undetected by an even moderately alert guard. Moving between shadows quickly and soundlessly is critical through judicious use of the dash command and avoiding breakable items in your path.
When detectable an ‘eye’ meter above each NPC in range slowly fills up and you are slowly given audible feedback in the form of whispering with gathering intensity. The more light and the closer they are the more quickly it builds up. If not in line of sight you can still alert guards by the noise you generate, although perks and liberal use of the ‘CTRL’ toggle can greatly reduce the noise generated (at the cost of speed in the latter instance).
If detected you then need to escape out of line of sight or otherwise hide, but beware that enemies will follow you to your hiding place if they spot you. These are some of the most intense part of the game, where the musical score ramps up and you're rushing to find a tunnel or ledge to make your escape. Be warned though, although there is no stamina bar there is a stamina mechanic. Run too long and you will run out of puff, slowing markedly and letting enemies easily catch you.
If there is no escape you can fight back if you choose, however doing so exposes the weaknesses of their combat system. At range you will tend to be sniped easily by accurate volleys from ranged guards whilst your own meagre supply of arrows is quickly depleted. In a melee combat is haphazard thanks to poor enemy AI and few tools you have at your disposal. Furthermore multiple enemies are deadly even on medium difficultly, while single guards could prove more than a match on the hardest setting. Stealth is the best policy, if for no other reason than the game just does it better.
If you can't plot a route to avoid an enemies patrol individuals can be taken out with the blackjack takedown and then hauled to a hiding place. If the body is found however you will put the enemies on alert, so take care using this method.
Patches of illumination can sometimes be turned off by use of a switch (if electrical), water arrow (if fire) or quick wave of the hand (if candlelight). Note that a change light levels around them will put guards in higher states of awareness, making you more likely to be seen. They will also often turn the light back on if the source is close to them. If necessary therefore it can be a good distraction.
Speaking of distractions, you can also use throwables or environment effects to divert the attention of a guard. However this tends to only attract one guard from a group, and will put them in a much higher state of alert, so use appropriately.
The tendency for enemies to appear in groups can make most means of distraction or elimination unwise. In such cases it’s best to just circumvent them where possible. Generally speaking the developers have made such paths available if you look for them, but they may have other risks or contain fewer collectable rewards.
At times the tension engendered by the game is absolutely epic, but it is undermined by the quick-load & quick-save of the PC version. For better immersion it’s often best to just use the autosave points, but you can sometimes be tripped up by a glitchy AI or LOS causing you to fail instantly.
All in all, it works adequately as a stealth but poor as a FPS where the lacklustre AI becomes all too obvious. For some the collection game will prove to be popular, but other will quickly plough through the story content and then set the game aside to gather dust.