I liked the general concept. The camera transition from a frog's view to a fly's view added a nice touch and enhanced the feeling that you were changing characters. The sound effects when explaining what to do after the frog catches the fly were pretty fun. It's a fun game!😁
I find it impressive when developers create their own game engine. It must require a significant amount of effort and careful planning to build a custom engine. Personally, I use the Unity engine, and I have no idea about the complexities of creating a custom engine but I imagine that is hard.