The city has been taken over? It's either the enemy is too strong, or the defenders are incompetent.
Reminder that when MC came to his Dad's aid. The enemy were THROWING DOWN BOMBS from a height magic nor arrows could reach and they were helpless! And Bombs were JUST INVENTED by the enemy, so that was it's first time being used in Combat to GREAT EFFECT! Not enough time for a countermeasure to have been made or set up (MC might have already thought it up, IF the fort was under his command! His dad though likely didn't have enough time to think it up, especially adding his shock and embarrassment about being saved by the "son he all but didn't say he casted off hoping he'd die quietly in a backwater previously USELESS as far as he knew location!" Remember he had egg on his face since he KNOWS in a sense, his other son has more right to MC accomplishments then he does, seeing that he was the reason he didn't KILL MC ON THE SPOT back in chapter 1)
The Third Option is, the "impregnable fort" finally met an Attack it couldn't stop because it never knew it EXISTED! Again, the Bomb Raid was TERRIFYING because they had NO DEFENSE AGAINST IT! MC was able to take the flyers down, but it's STILL a glaring weakness now that the possibility exists! Plus it hasn't been that long since the last attack! No one was aware till that stupid Prince's attack on MC Town that this was a full on INVASION of MC's dad's territory that was WELL PLANNED as it was meant as a STAGING POINT for the invasion of the Kingdom as a whole!
The one Spanner in the Works, they never factored was MC, but to be fair, the common knowledge was Production magic was "useless" because no one ever invested in it or truly understood how it worked (so LITERALLY it's "Right power for the RIGHT MINDSET that is MC" that it gets to SHOW how OP it is! Reminder that MC was able to show his Fiancee and everyone "just how POWERFUL Puppet Magic can be in the right hands and right equipment as well! That one ALSO having a logical reason of people getting stupid just cause ONE ASSASSIN with it used it! Not a surprise assassins used ELEMENTAL Magic, but THOSE got a pass because people saw the use in other ways before Assassins would use it, but cause an Assassin used Puppet Magic to kill FIRST before anything else! People went FULL IGNORANT! Speaking of which, can't WAIT for MC to meet his in-laws and show them just how BADLY they missed out on Arte like the assholes they are! What they thought was marrying her to some backwater fourth son thrown away instead had her a FUCKING FUTURE BARONESS and well respected, and they SHAMELESSLY know they have no right to make ANY CLAIMS seeing how unsupportive to out right ABUSIVE they were to her!) and it's funny since his dad never factored their tactics, but then they never factored MC either!