read the title and you’ll understand.
I read the title and didn’t understand.
Each level is either for practice or to teach something. The non-subscriber levels are so few (for each topic taught) that I would see all as required.
What brings you to the idea it isn’t?
read the answer and you’ll understand
But do you have to do it to get to the mountain world?
Also i am a subscriber
Technically no, as you can simply use the link to a later level (subscriber or not), beat it and continue from there.
I still recommend against doing so.