Isekai Maou to Shoukan Shoujo no Dorei Majutsu - Vol. 20 Ch. 96.2

[MangaDex]

 

[Strongstache]

If all it took to take down the barrier was digging underneath the wall's foundations to cause it to collapse, wasn't there a more practical method to doing so than that magical Big Bertha?

 

[Axccel]

They already showed us early in the series that the barrier extends outward, didn't they? That would mean this tactic simply could not work. Also, the damage was all from the magical attack that the barrier was showed able to block, which means the damage simply could not have extended far enough to endanger the wall.

Bad writing. Ignoring what you already established, and even what you just freaking showed us to railroad the plot.