I really enjoyed the game - I'm already a fan of tower defense-like games. It's nice to see how the thieves always take the fastest path to their gold (I assume you used a specific algorithm for that). However, the AI of the guards is still quite basic - you probably didn't have enough time to improve it. As a result, I felt that I could only solve Level 1 in a certain way.
Unfortunately, I couldn't progress beyond Level 2 because I couldn't place any more guards (I think it's a bug?). If the game had more diverse guard types with interesting AI, I can imagine playing it for a good while and having fun. :)