Haken no Kouki Altina - Vol. 4 Ch. 18

[MangaDex]

 

[Mathiason]

Thanks for the chapter!
Looks like the plan is underway. Although it's good the fort's commander can keep calm but too routine can be bad as can react clumsily to something outside the routine. Bronze knights are too impulsive for achievements and too hot blooded to think properly. They're facing an enemy known for defeating a force six times their size before yet are thinking nothing of it and have completely forgotten that rushing to put the enemy within range of their weapons also means they're in range of the enemies' weapons.

Spoiler

This is just a guess, but the comments about the soldiers having trouble sleeping due to all the cannon fire, there being mines, and needing gunpowder to excavate tunnels is probably all connected to Regis' plan. Could be wrong but I'm betting that they're sneaking into the bottom of the fort through the mines while blasting open tunnels using gunpowder. The reason nobody in the fort is noticing is because they think any blasts from the mines is from the cannon fire on the surface. Hence the cannon fire on the surface isn't to damage the fort but cover up signs of tunneling.