Handling edge cases

"My fortress is impenetrable," the man declared with unwavering confidence. "No enemy can breach these walls."

"What if a spy, disguised as a wandering traveler, opens your gates in the middle of the night?" his companion asked.

"And what if an enemy poisons your water supply with a slow-acting toxin?"

"And what if an enemy bribes your own soldiers to mutiny and open the gates from within?"

"And what if an enemy uses a trebuchet to launch diseased animals over your walls, spreading illness among your people?"

"And what if..."

"Enough, man, I get it," the man interrupted.