@Akurih yes.... in the light novel they show how they are operating.
1- they operating in shifts when protecting the walls so they can always be at their best condition.
2- what left of the main building turned into a hospital where the healer and her assistant stays. and they give priority for solders so they can get back to fight right away.
but world hate the mc they not gonna make things this easy... the reason is the monster in the last panel...