I've been picking away at the missions over the course of several weeks and I have to say this is one of the greatest stealth experiences EVER. The randomization keeps things fresh and the different layouts are varied and fun. There are so many different ways to play and the achievements really keep you coming back.
After beating the last level on Professional I have to come back to congratulate the programmer on an awesome job well-done.
I have two suggestions for improving this game, but short of that I don't think it would be possible for this to get any better.
1. Make a mode where you can only see where your mouse is pointing. This way the guards aren't the only one with a limited field of vision. Basically the player would have to move the mouse where they wanted to look, and they wouldn't be able to look past walls or other obstacles. Guards' field of vision would not appear unless the guard themselves were also visible.
- Ideally, the above suggestion could come with a better system for using the speed and invisibility boosts. With WASD, you could hold shift while moving to dash, and hold shift while standing still to become invisible.
2. Make it so you can distract guards. Maybe clicking on the screen would allow the player to throw a rock which could fall or collide with any walls it hit. When used point-blank it could be the same as knocking on a wall.
Anyway this is a really amazing game. Full points for this!