The physics of the game is all done in a 2d mario-like world. I then transform each tile and entity to wrap around a circle, using basic trigonometry.
Hi! The collision and physics is all done in an invisible flat 2d world (like the original mario). The actual tiles and sprites are then wrapped onto a circle -- it could be done in a vertex shader but we just do that transformation on the cpu. The tiles need to be deformed otherwise there are gaps.
We've fixed many of those issues and will upload a new build very soon. Thanks for playing!
As for the source code, we'll see what happens. If we don't continue developing it then we'll probably release the source code.