Thanks a lot for your kind feedback! I'm glad you enjoyed the game.
Actually I've never played Rayman but I think I'll have to give it a try to see the similarities 😅
Regarding the portals: I already tried to describe it in an answer below. Let my copy it in here:
I used ShaderMaterial for this. First I made a copy of the background image from each area and resized it to 128x128px. Then in the shader I calculated the distance(radius) from the center and rotated the pixels/fragments this radius (this gives a spiral) * sin(TIME) (this unwraps/wraps the spiral over time) + TIME (this rotates the whole thing constantly). I also set the alpha channel depending on the distance from the radius, this makes it a circle). You can find the shader here in the sourcecode: Portal.tscn
Feel free to use it if you like it, I licensed it under MIT.