To be honest that TNT he used to initially excavate is the exact answer to their problems if ever the kingdom invades Omui.
It's the exact thing that Nobel invented to make mining easier and also accelerated conclusions to wars due to its destructive nature when it was weaponized (it is also the reason why Nobel dedicated his fortune; establishing the Nobel Peace Prize as a sort of a penance to making wars a bit more of hell for all the people in the world).