All right, this "jump into a wall" glitch is turning out to be useful.
At least, when it's a thin wall, and there's room to run and jump and fall through, and it's on your left. For some reason it never works for me on walls to the right; I imagine they did the collision detection on left edges better, since most of the locked doors have to block you from the right. And don't jump through the wall shown in the picture before grabbing the treasure, because you won't be able to get back to it afterwards!
Oh, but I found an even bigger secret while labeling the map that's starting to drive me crazy. See, I was trying to find out the purpose of that one key that's directly in your path so you can't avoid collecting it. It seems silly to me if it activates something up ahead, since there would be no way not to activate it and you would never see the thing unactivated and never know what the key did. So I backtracked all the way back to the beginning, looking for any new open doors, since the key made the door opening noise. And at the very beginning, I found this!
It's a flippin' WARP ZONE! You walk through the passage there, but you suddenly end up on the ladder here!
What's more, you are automatically given all the treasures up to that point, so you can continue on and still finish the game!
You don't gain any score, though, and in fact, all the previous treasures and barriers are still in place. They really should have put a wall or something to keep you from backtracking from the warp destination, because if you fall down, you can't get back up, the door you haven't opened prevents you from going further back, and if you collect the heart or the diamond, you set your respawn point in that same trap, and the diamond gives you a glitched negative total of diamonds. However, there IS a way back up, just barely by using the glitch jump to get into the wall and jump through it back on top, and then you can use the platform the key starts to get back some more.
So I kept going from there, passing through more doors from the wrong side by glitch-jumping, and seeing the tiles the negative diamonds gave me. In-game, they first show a blank space, then the letters of PAUSE in reverse, then an arrow, then a piece of Robin's boot. On the totals screen, they go from a blank space to some punctuation to the last couple of letters of the alphabet. But when I got to my first negative crown, I ran into a problem.
I cannot get Robin to pass through this wall. When he gets far enough in that he would normally be able to exit on the left side, he gets stuck with no way out. If I make him duck, he never stops ducking, no matter which way he's facing. So the crazy fun ended there.
Oh, but did I mention? It wasn't actually that mystery key that triggered the secret passage. See, I tried loading some earlier savestates, and it turns out that I had somehow opened that passage much sooner. Savestate 3 was the earliest one where I had already opened the passage, while savestate 2 was the latest one where it wasn't open, so I uploaded both for anyone to play with.
http://www.box.net/shared/uuaq6z8252
http://www.box.net/shared/y1vecvczvk
Savestate 2 must have been when I first climbed the first ladder down, and savestate 3 must have been when I was about to test how far down the chain I had to climb to activate the next regular secret passage. At that point I had collected the previous treasure chests, defeated the guard, and blocked the mace ball with the secret spot from the other chain, but I hadn't yet grabbed the key underneath. This was when I was jumping all over the place to try to find more secret spots, so that must have been how I accidentally activated the warp-zone secret passage, but I tried doing all that stuff from savestate 2, and I haven't been able to open the passage again! So now I'm thinking I must have hit something REALLY secret that's almost impossible to find, maybe just a way for testers to skip half the game. Could it require touching a secret spot I forgot to try agin? Getting wedged in a certain wall? Shooting something with an arrow? Riding a ball somewhere or messing with its entrance area? Waiting some amount of time in some spot? Revisiting certain areas? Collecting certain items in a certain order or timeframe? Taking damage from a certain enemy, or not taking any damage for a certain amount of time? Doing any of those actions a certain number of times in a row? A secret button code? Oh good grief I hope I didn't enter a button code by accident...
That's why I call on all NES experts to help with this! I did some RAM searching, and I think 064A corresponds to the state of the secret passage. In states where it wasn't open, 064A was -1 (FF); in states where it was open, 064A was -65 (BF); and when I loaded a state from very late into the game, 064A was 127 (7F), which probably means "Keep the passage sealed, you won't need it anymore." (On another version of the ROM, I had savestates where 064A was -33 (DF), though that was when I was going for least treasure possible.) When I cheated to set 064A to BF, however, the passage didn't open, and it went back to FF as soon as I turned the cheat off, so it may be an expression of the secret being unlocked rather than what decides whether it is. But if it works right, it may save you the trouble of having to run all the way back to the beginning whenever you want to check to see if you've done it!