Really cool idea! I've actually always wanted to make some sort of "Security" game where you set patrols and guards in certain locations to defend something. I was excited to see this! I think this was really neat and a great starting point to something like that. I was pretty happy playing it!
I did think it was a bit annoying that the guards view decreased each level. Felt like almost a punishment for winning. And by the last level the routes the thief's took seemed fairly random so it felt a bit up to chance. I'd think it would be more awesome if as you went through the levels, maybe you got different types of guards, or even just more guards that 2 would be nice! Then you can increase enemy count or where they come from, etc...
Besides my minor gripes though this is still really cool. Let me know if you ever make this into a more full version! Great work on this!!