uh ? excuse me but how much sense can this have ? If you're fighting a huge army with low numbers and with offensive tactics, then you have MUCH more to worry than when you fight with defensive tactics, that's exactly the problem with Nox Noctis which I mentioned before: when a units defends it can kill more than an opponent per turn (drill works wonders), while an offensive unit most likely can't. So when you're facing bigger numbers it's much safer to fight on the defensive and I don't see how it could be otherwise. Also your supposed tactic looks just useless in the end:
So, answer me this, when you get attacked by a stack of say 40 units, do you just move all your guys into the closes city and wait for it to move on?
What if it has catapults, or Pyre Zombies? Putting all your eggs in one basket while they siege your city is a really bad idea, because stack damage, and collateral damage are very strong.
If you have units that are strong, and try to just fortify them in your city, they might attack somewhere else. Raider trait is a . .. .. .. .. .. Further more, against enemy stacks with a lot of weak units, or equal strength units, you could do massive damage to it while it walks (non raider) through your empire, except that your victorious warriors will end up being killed on counter attacks most likely.
1- fighting on the offensive outside cities means you'll have to heal after one fight, likely heal more than if you defended, and with less boni due to being out of a city.
Sure, if you are just going to have big stack vs big stack, it makes more sense to be on the defensive, unless they have siege, which can greatly modify this equation, because they have high withdrawl and collateral damage.
2- some targets just can't be killed on the offensive due to their defensive boni.
And some armies are too strong for your defenses, this doesn't seem like a real talking point.
3- inability to use your defensive boni.
Defensive Bonuses, yes, I believe you've mentioned that already, in this list, in both of the proceeding two items.
4- so you will manage to kill some of the attacking stack. This however won't stop the rest of it from pillaging all their way through your nearest city, which will be where you will likely stop a bigger enemy stack. However, without Nox Noctis you could have achieved this much before, and you could have used high mobile units like horsemen to weaken the stack without counterattacks in the same way you'd do with Nox Noctis. Sure, it's nice to have invisible units, but not if the drawback is bigger than the advantage.
Woa, now we are just completely theorycrafting. We have Nox Noctis, and horses that won't lose, and we assume we could have just fortified on the border and they'd have attacked us there. This is just fantasy. Large AI stacks walk right up to cities, and ignore most things else that aren't easy prey, and then kill the city, it is how they are programmed. Are you going to defend each square with fortified units? Put them all on the border?
What if, dare I say, you've got units that aren't good at defending? Horsemen or Assassins, or whatnot. Fortify those in cities two?