people seem to be forgetting that this series keeps throwing in time skips between chapters, usually small ones, days or weeks, occasionally months...there's no sign that this happened immediately after they finished setting up the defenses, it's just that there wasn't anything important to MC that happened in that time gap
this isn't even the worst series I've read in the same genre for inter-chapter time skipping, some will show MC going into months/years of training at the end of one chapter and coming out having mastered several new techniques at the start of the next