I completely agree with the OP here. To me, the inverted castle just seems like a cheap way to extend the time you have to put in to complete the game. Sure the enemy placement is different and they added more gear to find, but I can't get over the fact that I'm just exploring the same exact map I'd already explored prior but now rotated vertically. On top of that, the fact that they repeat the same 2 or 3 songs throughout the entirety of it really started to test my patience.
Honestly, I kind of feel the same about the "extended maps" in the other IGAvania games. You know, the palette swapped stages using tilesets from other parts of the game? Never particularly liked those. In my opinion, it's also a glaring issue with Order of Ecclesia as half of the stages you explore recycle not only enemies but tilesets as well. I understand the developers have a finite amount of time and resources to complete the game and so they cut corners when they have to, I do. It doesn't kill the experience by any means at all but I do see it as a flaw, myself.