Hi ! I also discovered the game on pico-8. I thought there was a good potential, but actually it's even better than I imagined !
I like that additional characters and the merchant allow to better control the gold income and thus the level progression of the heroes. Getting gold on lower levels becomes more interesting because it can be spent on merchants.
This motivates the player to clean a level instead of rushing, which adds variety. The prisoners system does that well too.
I love the battle against Archeos, it's very dynamic, and there's reason to keep finding keys until the end.