How do I close the gap at the west wall in a Spires castle?

[Tarragon Slayer/MZB]

The title says it all. I tried placing a stone paver, moving that up to the third floor, then placing a wall on top of the paver... So far so good. Then I tried to lower the wall (first separately, then while keeping it on top of the paver). That still works but only until I reach the floor of the third floor. After that, I get the message that I cannot lower it any further.
I'm rather certain this works in a classic castle. Perhaps it doesn't because this is a Spires? If that's not the reason, what am I doing wrong, please?

 

[Zynia]

I don't think you can because the outer wall east of the gap should be a southeast corner wall to connect properly.

Solution wise, you can try an crafted stone arch instead of a stone wall to make it a doorway.. the gap would definitely bother me too lol

 

[Archangel0]

did you try putting the wall piece inside a bag and axing it?

 

[Grugot]

It can't. You need a corner piece as stated by Zynia.

 

[Petra Fyde]

does this help?