This area only opens up after you've beaten the Elite 4 and gotten the National Dex. it is off of route 214, which is near Azelf's cave. there is no specific order in Turnback Cave, and every game has a different configuration for it, which is why i did not show my way around it. you have to just keep looking for those pillars, and when one of them has a 3 as one of the numbers, go in any door besides down. down will take you to the opening of the cave. also, you cannot use escape ropes in here. you'll need a Pokémon that knows Rock Clime, with Rock Smash and Defog if you want
