reverse Tower Defense feels remarkably similar to a regular game in this genre. The fact that the enemy is also upgrading as you progress forces you to make quick decisions: upgrading your heroes or send the next wave. In terms of tactics, I realized that I had to intelligently move Namgets 2D and 3D, so that Bon Bon would fire on one of them, thus allowing the others to run to the end. So I could only pass the first level without cheating. On 4chan was quite lively discussion of tactics to pass, so to finish all levels is realistic.