In this video, I finally figure out that there are two doorways near the bottom of the place I was stuck in. After discovering this, I'm able to progress.
I also did some farming for rare items around here, including amulet helm and guardian ring. Thanks to Icleus of the Steam Community for providing a helpful guide on rare and missable drops of items:
I also take out the boss here, so we can progress to the Pure Lands. I might also want to add, that Neko sells some good armor outside of the palace, if you backtrack around the outside of the place. You will need to purchase or own at least one of each equipment type to get all trophies (or achievements, if you're playing a STEAM or different version).
There are 137 views in 14 videos for Secret of Mana. About 12 hours worth of Secret of Mana videos were uploaded to his channel, roughly 6.46% of the content that Neon Shark has uploaded to YouTube.