The golden coins are definitely going to be an achievement hunt. They are all gatherable, but not always in an obvious way.
The collider on the spikes is a triangle slightly slimmer than the spikes, You should not be able to trigger a collision unless you're touching a spike. I'm going to see if I can make the spikes a little bit more menacing.
When you enter a portal, you currently get 2 randomly selected levels from that particularly difficulty. I was thinking about making the player start out with 2 levels per run, but adding 1 for every level of new game+ the player has achieved. Making it harder and harder to (re-)finish the game. You would also start off with just one life and have the ability to buy more by visiting randomly populated shops that sometimes get inserted into your levels run.
Incidentally, it's not the wizard who sends you off, but Greg, the gatekeeper. Have you paid him yet?
Thank you for your feedback!