Hi again,
I'm assuming by map you mean the pause screen and you're right there is a typo there, there are 18 collectables in total.

As for the one in the 3 towers, my first hint is you don't need the magnet.
My second hint is, its on the right side of the level (i.e below the water tower area).
If you need/want more hints, let me know.

Well, I just replayed the whole part again and still couldn't find the secret passage.

On a side note, I noticed that some secret passages are clearly visible on the map (the wall is different) while others are not (the walls are the same everywhere).

On another side note, if I activate the cheats, will it prevent me from obtaining trophies ?

I'll narrow it down for you more, from the first main scene of the castle, go right one scene, it's there!

Another hint for you, a long dive is required.

You are right, the entrance to all the collectable areas is intentionally marked with a different tile, as a hint. The one you're missing is also marked that way!

No, the cheat will not prevent you from getting achievements or any other limit.

I don't understand what you mean. Are you speaking of the big open part with the clouds that you need to traverse to get from the central part (with the door to the Foyer) to the right part (under the Water Tower) ?

And when you say "a long dive", do you mean that I need to jump down from the top of the Water Tower ?

Also, I noticed a long time ago that the entrances to secret areas are always on tiles that look like the inside of the wall. My point was that, sometimes, the overall map shows it (The Tower, Master's Den) and sometimes not (Tower of Pain, for instance).

Speaking of which, is the secret passage for the special coin also marked with a different tile ?

--> Edit <--
I found the HP upgrade in the wall of the Water Tower but still no collectible. :-(

Yes, the one with the clouds. no need to go to the scene above it (top of the tower)

Another hint, you can see that area from the very beginning of the scene (left side).
If you won't get it still, I'll send you a screenshot :)

Oh I see your point about the map, I'll go over them again.

No the wall leading to the coin is not marked on the map, though it is the center wall tile too.

Finally found it !

My mistake was that I was looking for a much more obvious difference in the tile style (like the other secret passages).

Thank you so very much.

Cool, I'm glad you finally got it!

You are welcome, cheers!

Bonus question: what do you mean by "center wall tile" ?

Like you said, the tile that looks like the inside of the wall, no flat surface.
In this case is the 4 small squares tile, same one you saw when you picked the last collectable.

So, I suppose that means it's either in the Three Towers or in the Tower of Pain.

I'll look for that. Thanks again.

I give up. I've looked everywhere and still couldn't find the special coin.

I even cheated to get the jetpack and couldn't find the secret passages detector.

Besides, I couldnn't find some of the coins either and I'm utterrly unable to perform the insane jumps needed to get some others (hello coin at the top of Tower of Pain).

Playing the game to the end of the story has mostly been fun but trying to complete it is not.

Ha, I'll give you some more tips :)

The upgrade machine you are looking for is somewhere in clouds at the top and left of the tower of water, but it's kinda useless to you since you got all the collectable already.

The special coin is somewhere inside a cluster of clouds between the tower of fire and tower of pain, you can see it from outside, but you can only access it from the tower of pain, when you find it look what scene is next to it on the map.

The coin at the top of the tower of pain is hard to get, probably the hardest, since it's the last one of the game. But if you managed to get to the collectable then you know how to do it, just a few more steps of the same method.

Can you tell me which coins you can't find (areas name on the map).

OK, I finally got the rainbow coin. Never would've found it by myself.

Congratulations!

The special coin and upgrade machine are just a little extra hard to get secrets.
They don't hold any weight in the full completion of the game (you can get 100% without them) and you don't get any achievements for them either.

Frankly, I didn't expect anyone going after them, at least no for a long time after the release.
You caught me by surprise :)

Well, you said yourself in your guide that getting the rainbow coin means that all remaining coins are shown on the map and getting all the normal coins IS required for a trophy.