public static function processSiege(Army $army, City $city)
 {
     // TODO check if the army has catapult
     // TODO message users about the result
     $attack_point = $army->calculateAttackingPoints();
     $defense_point = $city->calculateDefensePoints();
     if ($attack_point > $defense_point) {
         // attacker wins
         // the city becomes the attacker's city
         $city->update(['owner' => $army->user->id, 'capital' => 0, 'nation' => $army->user->nation]);
         $city->hex->update(['owner' => $army->user->id]);
     } elseif ($attack_point < $defense_point) {
         // defender wins
         $army->destroyArmy();
     }
 }