Ah I see now! I tried building the wall somewhere else, then selecting it with Right click + Shift and copying it, but I did not continue to hold shift as I copied it.
The building now looks as it should, thanks everyone :)
I'm trying to recreate a building I found in one of the sample maps, but for some reason I can't get the walls right.
In the image I posted you see two buildings, the upper one is from the sample map, the lower one is my creation. As you can see the walls don't align properly in the lower...