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:
[Difficulty] -- THIEF 3 AI visual acuity multiplier - how well AIs can see Difficulty_AIVisual_Mult_Easy=0.9 Difficulty_AIVisual_Mult_Normal=1.0 Difficulty_AIVisual_Mult_Hard=1.15 Difficulty_AIVisual_Mult_Expert=1.2 AI auditory acuity multiplier - how well AIs can hear Difficulty_AIAudio_Mult_Easy=0.8 Difficulty_AIAudio_Mult_Normal=1.0 Difficulty_AIAudio_Mult_Hard=1.03 Difficulty_AIAudio_Mult_Expert=1.13 AI tactile acuity multiplier - how well AIs can feel Difficulty_AITactile_Mult_Easy=1.0 Difficulty_AITactile_Mult_Normal=1.0 Difficulty_AITactile_Mult_Hard=1.0 Difficulty_AITactile_Mult_Expert=1.0 AI hitpoints multiplier - all AI starting hitpoints will be scaled by this Difficulty_AIHitPoints_Mult_Easy=0.75 Difficulty_AIHitPoints_Mult_Normal=1.0 Difficulty_AIHitPoints_Mult_Hard=1.2 Difficulty_AIHitPoints_Mult_Expert=1.3 AI combat speed multiplier - all AI combat animations are scaled by this Difficulty_AICombatSpeed_Mult_Easy=1.0 Difficulty_AICombatSpeed_Mult_Normal=1.0 Difficulty_AICombatSpeed_Mult_Hard=1.1 Difficulty_AICombatSpeed_Mult_Expert=1.2 AI-to-player damage multiplier - all damage applied to the player by the AI is scaled by this Difficulty_AIToPlayerDamage_Mult_Easy=0.5 Difficulty_AIToPlayerDamage_Mult_Normal=1.0 Difficulty_AIToPlayerDamage_Mult_Hard=1.25 Difficulty_AIToPlayerDamage_Mult_Expert=1.5
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.