The staircase design on the 20th floor is intentional. In the original version, there’s only one save slot, and once you enter the 21st floor, you can’t exit. This setup ensures players get a fair shot at defeating the boss on the 21st floor. Plus, there’s a hidden floor after the 21st. If players could submit the cross later, it would make the hidden boss easier to beat.
I love TotS for its minimalist RPG style. Actually, I’m planning to create my own version of TotS with a major overhaul of the combat system. The challenge will come more from using relics and floor layouts rather than just numbers.
Thanks for your reply, it really motivated me!