We have just released a patch to fix an issue on level 15. The cannon tutorial was not working properlly, making it impossible for the player to progress.
Please grab the latest update, if this is still a problem after the update, or if you encouter any other issues, please let us know and we will fix it ASAP.
Thanks "Singularity Axiom╘[◉_◉]╛" for reporting the issue.