Loved the game concept. A very smart design for the brief of time. The sprites and presentation were nicely done. It was also challenging to play. I have a suggestion, although I don't know how hard will it be to implement, but instead of pre made levels do a procedural system where every time the level is beat, the grid increases and random number of buildings and rules are assigned to the new level (which may be hard to balance) and have a timer running down. Every time a level is complete increase and game ends when timer runs out.
Good luck!