Nobody seems to mention that problem so i do now
just take a look at the screenshot and you will see it on the right side -
i created this wall in one piece. also i didn't use the shift+f12 mode to create them!
i guess this problem might be caused becaused these walls were designed for flat terrain.
also what i want to mention here is that there is no dismantle function for basic walls - the only option is upgrading basic walls to defensive walls and dismantle them then.