Before you start calling out something as RAW, please make sure the rules are actually -written- somewhere.
I cited my RAW ruling. It's from the rules on Walls, which I will requote.
A wall fills a specified number of contiguous squares within range, starting from an origin square.
Each square of the wall must share a side—not just a corner—with at least one other square of the wall, but a square can share no more than two sides with other squares in the wall (this limitation does not apply when stacking squares on top of each other).
You can shape the wall however you like within those limitations. A solid wall, such as a wall of ice, cannot be created in occupied squares.
Notice that the wall specifically states how they work in a 3D environment with stacking. You can build walls upwards specifically in the RAW and it makes an exception allowing this. Nowhere in the RAW does it state that vertical stacking of a walls power is free. Nowhere does it state in the RAW that the vertical aspect of a wall doesn't count as another square - in fact the ruling right there by RAW shows walls assume they can be stacked in 3D space (forming well, a wall). And that each square of that wall takes up one of the squares you have to build it (implicit from the first sentence).
The first sentence is critical, because the rules are addressing how walls are formed. It states:
A wall fills a specified number of contiguous squares within range, starting from an origin square.
These can be horizontal or vertical. So long as they follow the rules there for walls - which also means they are limited by the number of squares in the power. Illusory Wall states it is an
area wall 8. It can therefore occupy eight squares. Not eight squares horizontally and 24 vertically.
Eight squares.
That is by RAW.
The idea of spells taking up 'cubes' when you go to three dimensional movement is not actually written in the DMG.
But is in the
rules that I have just quoted to you that tells you how a wall functions in 3D space, specifically with stacking the wall pieces together. So this isn't plain true when it comes to walls.
What does NOT exist is a mention on how vertical range works with Area effects, or bursts, or blasts, or walls.
I disagree, unless you have another interpretation as to what "Stacking" the wall is, it's very clearly telling me how you can build a wall in 3D space. It even makes a rules exception to specifically allow this.
Once you actually get into walls, you've gone well beyond the pale of Rules as Written, and are deep into Rules as The DM Had To Decide Them On The Spot.
I am quoting the rules for Walls to you. If quoting the rules for Walls, that tells you
how you can build 3D or vertical in this case walls is not quoting the rules as written, then what exactly do you want me to quote?
the 'rule' you are trying to invoke is non-existant.
I firmly believe this RAW below I've bolded completely supports my position.
A wall fills a specified number of contiguous squares within range, starting from an origin square.
Each square of the wall must share a side—not just a corner—with at least one other square of the wall, but a square can share no more than two sides with other squares in the wall (this limitation does not apply when stacking squares on top of each other).
You can shape the wall however you like within those limitations. A solid wall, such as a wall of ice, cannot be created in occupied squares.
So you can make a vertical wall that looks like this:
O=Origin square. X = Wall in 3D space (looking at it from the ground). W = Wall.
WXXW
WXXW
WXXW
WXOW
That's eight squares (area wall 8) and four squares high. The rules clearly define you are allowed to do this. Now let's look at it from a 2D view. With B = free square. M=Monster. X = Wall in 2D space this time.
WBBW
WXXW
WXXW
WXXW
WX0W
WMBW
If that was 2D, you couldn't do that because the wall is illegal as numerous squares share more than 2 sides. Again, that's by RAW, a limitation that building a vertical wall doesn't have (so this proves that the rules for walls have already addressed how you make a vertical wall and that vertical walls follow the same rules for horizontal ones, with a specific exception).
You could build a wall in this case that looked like this:
WB0W
WXXW
WXBW
WXXW
WBXW
WMXW
Because none of the squares share more than two sides.
Now why am I going into this? Because it shows that in all examples, you still can only build a wall that is the same as the area it gives within the limitations. In this case, it can be eight squares built either as eight total horizontal squares, or up to four squares vertically (enough to block a 2x2 corridor that is 20 feet high!). Nothing in the rules for walls says vertical squares are free or don't count as a part of the wall. Neither is the rules for making walls silent about making a 3D wall, because it specifically addresses it in allowing stacking. Implicitly, this means 3D walls follow exactly the same rules as a horizontal one and you can only build the wall up to eight squares in total (horizontal or vertical, with a 4 square height limit on being vertical). Not 32 as is claimed with illusionary wall.
Edit: The point is it defies the way Walls are written as working by not being an area wall 8. The interpretation that it is eight squares long and four squares high just doesn't match with the way all other wall powers work and the base rules for walls. The vertical squares of a wall are clearly addressed in the rules for building walls as being a part of the number of squares you're allowed to build them. Given that the sentence in Illusory Wall can equally be read to be a limitation on both the horizontal/vertical and not as "Horizontal PLUS vertical" and the RAW for walls there is a strong RAW
and RAI argument it is not four squares high by default.
Edit2: The other reason here is the way I read the power. The power already tells me how many squares it occupies (area wall 8 within 20). The text tells me the limitations of the horizontal and vertical. It is not telling me it's an area wall 8 + 4 squares of height per horizontal square.