Thanks for playing and glad you enjoyed it! Yeah, I shoulda made the secret passages even more visible - they've got an animation on the wall texture to try to call them out, but apparently it's too subtle ^_^. I did try to mostly just put them in consistent spots, but unfortunately they were probably just varied enough that everyone feels they gotta bump all the walls -_-. Merchant part was inspired a bit by Uncharted Waters (SNES one came out in 1993, but there's been a ton of them), and more recently Horizon's Gate which is like that crossed with Final Fantasy Tactics ^_^.
Underground was mostly just an excuse to have some variety and big open spaces, and started from a randomly generated dungeon that I hacked up to make the topology I needed. Great that you made it far enough to see that though, and congrats on beating the game!