An invisible wall is an outdated approach from an era when technical limitations prevented games from featuring extensive graphics. If you don't want the player to go in a certain direction, place a pile of boulders or a steep incline there instead. Invisible walls always break immersion—without exception. I once went caving in real life, and I have to say that you've managed to partially capture that genuine fear: as you descend, squeezing through narrow passages, there's always the dread of reaching a dead end—and that’s truly atmospheric.