I played all ten levels. The only thing that threw me was the word "enclosure" being used to describe what I'd call a box or a table or a square you want the characters to line up with. To me, the word "enclosure" describes the "play area" the characters exist and move within. It's fun as a jumping off point, but I'd want more moving pieces to keep playing further. Maybe not a 3rd character, like a square, but maybe puzzle tropes like push blocks, switches, or ways to temporarily change or swap the forms of T & Y. Besides that, I think a bit of animation can go a long way. Not necessarily "art", but easy stuff like making the characters slide from tile to tile instead an instant shift. Eye blinks. A "Congratulations!" text that flashes for a second before you see the level reward art. When you clear all ten, maybe let me see a slide show of the arts I unlocked.
Thank you for playing through all 10 levels! I'm glad the difficulty felt fair.
You're right about "enclosure" — I can see how that word could be confusing. I'll take another look at it.
I like your point about new mechanics rather than another character. Pushable blocks, switches, and temporary shape changes are all interesting. The slideshow idea for the unlocked illustrations is a nice one too.
Thanks for the thoughtful feedback — it helps me think about what could come next.