The game itself itself was not very exciting, but the concept is fantastic. The keyboard gimmick was really neat, but I'm glad that you also gave the option to use your mouse since the novelty wore off quickly. Being able to choose the path makes your options very limited as there's only one optimal path, but I understand that's what you were going for, and the challenge is to figure the one or possibly two paths that work.
The key counter at the top of the screen didn't convey the idea of how many keys I could use very well, partially because you don't know the limit until you've exceeded it, and partially because you can place as many keys as you want and the only indication is that the number changes color, which is likely to be missed entirely since nothing about the limit is explained in the help screen and the change from yellow to white is rather subtle.
I thought that I had hit the key limit once the number changed color, but I found it impossible to form a path within the limit in one level. However, I added another key past what I assumed was the limit and the simulation ran, which made me wonder if the color change was indicative of anything or if the only way to know you've hit the limit is to attempt to run the simulation and fail to start.
I feel the number should start at the maximum and count down to 0, and prevent you from placing any more keys then. The number becoming red would have been a better indication of being over the limit, since it stands out more and is generally associated with wrong things.