I had a great time with this game. I think the levels were well designed and had a good difficulty progression aside from the bug on level 1. (Funny that you missed an alternate solution on the simplest level and not the later ones. I was worried I would run into a similar bug later but I didn't.) I agree with Fruit Kobold's comment that the later levels were more straightforward, and I think you could make them harder by using less tiles as constraints. I would like to play more levels as these 6 were fun. The art was very good. I thought the audio could have been better though. I think the removal sound, the mars placement sound, and the ambient sounds for city and mars were a bit too loud or abrasive for the relaxed atmosphere of the rest of the game. Also, use different removal sounds for each world. I imagine that some ambient background music like in Mini Metro would work well with this game, but that's personal taste.