Very fun!
I definitely agree with a sentiment I saw glancing at the reviews that the image that explains the game's rules should be available through a button at all times, I just took a screenshot of it and kept it on the other monitor for a bit until I understood how it all worked.
I appreciate the unique take on the "echoes" theme and I think it fits pretty well for that.
Overall creating a game with mechanics this intricate in just 15 hours is very impressive and I could definitely see this becoming a very popular strategy game if you continued development. If you wanted to take this further I feel you should put more focus on the visual and possible story aspects of the game and of course mutate and evolve the gameplay as the player progresses adding new mechanics the further in they go. Overall I do hope you continue development because I'd love to check in on this and see how it progresses!