Thanks for your feedback! I actually agree that I didn't make that as clear as it should be - you can press S to see the layout of the tiles in the 2nd dimension, but I think it's quite a difficult thing to convey in a way that's easy to understand.
I was actually thinking of making another prototype based on this where the portals would just flip everything inside them (e.g. empty space turns into ground and vice versa) - I think that would be easier to understand tbh.