Invulnerability, and blind guards
This requires a bit of code editing, so do the smart thing and BACK UP YOUR FILE.
Now, that being said, you'll want to find the Thief 3 directory and go to the System Folder. (Default is: C:\Program Files\Thief - Deadly Shadows\System). Look for default.cfg. Open up this file in your normal text editor and look for this:
-- THIEF 3
AI visual acuity multiplier - how well AIs can see
AI auditory acuity multiplier - how well AIs can hear
AI tactile acuity multiplier - how well AIs can feel
AI hitpoints multiplier - all AI starting hitpoints will be scaled by this
AI combat speed multiplier - all AI combat animations are scaled by this
AI-to-player damage multiplier - all damage applied to the player by the AI is scaled by this
The higher the number, the better they see, the harder they hit, faster they are, etc. Edit the numbers to your liking, start a new game and enjoy.