As far as I know, on the Mac version at least, one-way walls can explode from any position.
I have checked that the same is true in the PC version and the SNES version.
But I did some more careful testing as well, which I think revealed why Peter had the confusion with one-way walls (below are the notes I pre-typed before making this post):
--------------------------
There is no inconsistency as far as one-way walls themselves are concerned.
But there is some "inconsistency" instead associated with steel areas.
One-way walls are
always explodable.
However, there is a general interaction between steel areas and exploders. Namely, the game takes a shortcut. You might think that the game will actually mask out the exploder's bomb range area with the nearby steel areas, in order to prevent you from exploding steel.
Well, that's probably too slow to do for the PCs back then, so instead they did this (or at least that's what I believe they do): when the exploder explodes, they check if it is standing on a steel area or not. If yes, then they completely disable the taking out of terrain pixels when the exploder explodes. On the other hand, if you are not standing on a steel area, all terrain pixels in the explosion area will be taken out, even those that are part of steel areas (the steel area status itself is still intact I think, just the corresponding terrain pixels are removed).
This leads to some remarkable effects. Fire up Fun 11.
Try putting a stopper both on the right (next to the one-way wall's left side), and on the left (next to the vertical bar on the very left). Now explode each stopper. Notice that the exploders explode, but in both cases no terrain pixels whatsoever are taken out. This shows that it is an effect of standing on a steel area, rather than any side effect of the one-way wall, since the left stopper is nowhere near the one-way wall, and yet his explosion is disabled just as the right one is.
But now, for more fun, try this: make a lemming build just one step immediately left of the one-way wall. Now put a stopper on top of that step. Now explode it. Because of that extra step, the stopper is no longer standing directly on steel, but instead got one pixel of vertical separation from the steel area. That is enough to cause the game to apply the full destruction of the explosion to the surrounding area. As a result, parts of the one-way wall got taken out, as well as even some of the steel-plate terrain piece.
So the question now is, should we fix the game mechanics so that terrain pixels in steel areas are properly protected from explosions (and conversely non-steel areas are not protected merely by having the exploder standing on steel), or should Cheapo employ the same shortcut PC Lemmings took (if it hasn't done so already), and accept all the resulting inconsistencies?
It is interesting to note that although it is conceptually a bug, it seems clear that the programmer intentionally decided to make things work the way it does, as I said probably for efficiency reasons. So it's even debatable whether to really call it a programming "error" or not.
---------------------
If other versions of the original don't allow one-way walls to explode, maybe you should consider having two types of one-way wall, one explodable and one not.
Well, I've just seen that 2 other versions also behave identically as the Mac, so this is not an issue. I'd definitely shy away from having multiple types of one-way walls, it just gets too confusing for the average player.
