Difficulties, and possible improvements.
For me, there are three things that create real difficulty in the game.
1) The speed that the turrets (sentries) rotate. There is a very small time span in which you can pass them. This means that people with slow reflexes can -never- complete the game. If you included difficulty settings, where the sentry rotation was slower, you would give a chance to slower players. Alternatively, you could perhaps introduce a non-fatal puzzle that gives the player a radio device that slows the sentries down a little. You could provide different paths through the game, for different player styles.
2) The Sentry's cone of vision - The left and right limits of the sentry's vision are clearly shown - they're hard lines of blue light. Outside the line, it can't see you, inside, you're dead. But the sentry's vision at the front just fades into a blur. It's not possible to know whether it can see you or not, until you die. If the front of the cone of vision ended in a hard, visible line, like at the sides, things would be much more helpful. I know it's not realistic, but if you want to make the game playable for more people, reduce the difficulty, and make it clear when you're visible, and when you're not. Maybe in the easy game, the player could find special glasses that do this. Again, you could make this an option on solving a puzzle. It would give an option to people who struggle with the game's play style.
3) Sentry's hearing. This is crucial - there's absolutely no indication of whether you can be heard or not. Yes, the character holds up her arms in areas where you need to sneak, but it doesn't give you any real indication of whether you can be heard or not. The only way you can find out if you've made too much noise is when you die. There should be some indicator of how much noise you're making, and how the sentries are reacting to it. Perhaps the player could carry a sound meter - possibly this could be an item you find early in the game. Maybe only in the easy game? Maybe a puzzle option?
I suggest that you don't be so linear. Give your players options. The puzzles you've made are good, but they shouldn't be the only way through.
The more paths through the game you provide, and the more you appeal to different player styles, the more customers you will attract.