Hmm... I don't know. If you visited the red tent (which is in the sands near the eastern magic castle) and then defeated the demonesses guarding the gate, you should have been able to pass through the gate. After that, more demons and cave lords appear, and then there's a red hero who, due to his stupidity, drops the key to a new dungeon that will be unlocked on the map. It's that dungeon that leads through the green wall.
Thank you! It turns out I missed the entrance to the dungeon in the center of the map, where the demon hero is located :D Well, as they say, everything happens for a reason! :o
