Okay, a fixed version is up. The offending area is now blocked off, though still visible for the sake of intrigue ;). The cave entrance is also slightly repositioned, but still on the same screen.
Viewing post in Quest for the Master Sword comments
Yeah, the dungeon does rely pretty heavily on trial-and-error since there wasn't space for complex mechanics, and storing two copies of one sprite and using tile properties was cheap. The entire map is shown for a frame at startup when it's being drawn, so your cheating isn't entirely unprecedented. Glad you had fun, and thanks for the feedback!