Im off for the night, but promise to take a look at them tomorrow.  To be sure I understand, when you say castle wall, your talking about arches wall_flagstone? 

First attempt was a big fat Fail.   


On to plan B:  Ill be cubing or halfing the cave walls and trying double and single-height crenellations.  Wish me luck!

Before I go and spend a bunch of time making them, I tried blending the images a bit better.  Let me know what you think.

I had to stop myself before I ended up making a map. 

Yes the seams are what catch my eye also, that and using only 2 kinds of walls for each side.  So by the time im done there will be 4 for each side, that should help blend them better.

Give me time, hopefully we will get some nice stacking cave walls yet.

Plan C:

Caves arent square, this is the problem with stacking non square objects hehe.

I know your request was for stacking cave walls, but this will take a bit more talent then I have.   Being a recent self taught Gimp user who cant draw, and doesn't have any art classes this is beyond me. 

Ive tried many things for those darn walls, but cant come up with a way to stack them nicely and without seams.

Not sure if I'm missing something here., but the OP talks about using walls for illusions. Ie, IIUC, placing ranks of walls on squares north and east of where you want the wall to appear so it looks very tall. IMO such illusions are damnable because they rely on players especially and mobs (and missiles and spells) being only in a specific location on the map -- or rather, not being in the wrong place --or the entire illusion falls apart and you have things moving through walls and so on.  Similar to illusions to make towers and cliffs and raised platforms.

It can definitely be done -- Tower of Bebeniss, John Smith's desert, etc -- but IMO generally with too much mapper effort and player restriction to generally be useful.

In this case, there may already be a solution, alternative images.Here you make two PNGs with the a flag and the b flag (that is wall.a.111.png and wall.b.111.png).  Image a is shown when the wall is in the left, top, or right quadrant relative to the player. Image b when in the bottom quadrant. So make a nice and tall and b single height/transparent/whatever. Then just place your arch normally, no illusions or trickery needed (and for the artist no mucking about trying to get the images to look blended.

See newarch palisades and honeycomb walls IIRC for some examples.