Assuming "modern weapons" means light arms carried by modern infantry: These are different from a castle. If the enemy makes a mad rush for the breach, they're

Assuming "modern weapons" means light arms carried by modern infantry: These are different from a castle. If the enemy makes a mad rush for the breach, they're going to get cut down by your guns, so the walls are more to provide cover from enemy fire than to physically keep them out. Assuming "modern weapons" means bombers, tanks, cruise missiles: this is not going to stop them at all.