I haven't found that room yet. I'm still going to explore the whole game regardless, and try to get everything there is to get. Based on the map and going around, I figure there are probably as many bosses as there are unique areas (water, factory, lava, forest, castle, etc), give or take a few. I was mostly surprised that beating that boss sent me back to the title, which confused me when it comes to the coins, as I thought I'd just collect them as I went. Now I'm thinking there's a specific purpose of the coins for some particular boss or room (or maybe they might have something to do with the pillars?), since they'd have to be collected in one run, and it seems like the game is meant to be played in multiple runs, rather than a long play.
I'm not trying to get spoiled or anything, just making sure that there's nothing obvious that I missed. I'm assuming the things that are still mysteries to me are supposed to be until I figure it out.