itch.io

Download it, view your downloads folder in the browser you downloaded it from, double click it. If you don't have WinRAR or 7Zip i'd encourage you to download those as well, when you double click the file from your downloads it'll open one of those programs, whichever you downloaded, then double click "killsonic.exe"

I come from BTG's YouTube channel, saw this and immediately downloaded it. I hope you find the drive to revive this, or perhaps allow others to implement it's system into other sonic level creators since this has a save/load boss function. Make a boss - make a level - load the boss into the level. Amazing stuff, super fun, hope you revive it.