So basically you're arguing that there's no need to build any walls at all, because they can be countered by using all sorts of convoluted strategies or magic...
And that there's no advantage in using delay tactics against any enemies, because they'll just magically invade from all sides anyway, and then carry enough water on them to counter any fire spells...
Why not just say that the enemy can fart lightning and is immune to any damage.
Dividing your forces and your resources is not exactly strategically helpful.
Because obviously all armies are always gathered in your castle all the time and there's no such thing as a surprise attack and buying time...
If you have a horizontal portal rather than a diagonal one then that is in no way the case. Firing at an ascending fire arc through the top of a door is a lot different from trying to fire through the door at a descending fire-arc.
Why again would catapults on one side be forced to fire at a different angle then the ones on the other side?
And if it is a 1 way portal then there is no way to retaliate at all. They can bombard you for all eternity and there's nothing you can do about it.
Yesh, except know that an enemy force is going to attack soon... forts have been build for exactly that, and that's in places where there's no magical portal to bring enemy armies to your doorstep from miles away...
Because otherwise the whole wall will collapse burying the entire army defending it. Why spend all that money on building walls when you could instead build a bigger army or better fortifications for your towns.
Because it's cheaper then having to feed and house thousands of soldiers every day while they're all in the same place too, when you could allow them to go about other business while you're not under attack, and then when an enemy army tried to invade you'd have time to gather your forces...
There is obviously no need to cross the rubble at all. One you've buried the defenders beneath the rubble you can just go around it.
How do you go around rubble from a wall that pretty much covers the whole area where your army is coming out of (the portal)?
And nobody's yet considered the possibility of carrying buckets of water through the portal so that the moment that the mages cast their firewall spell then those who have already got through the portals all (several hundred perhaps) throw said water buckets at the firewall.
Not any more then you considered the possibility that one might cast the firewall spell in the middle of the enemy, or that if having a bucket of water negated the spell it would be a rather useless spell in the first place...
You outnumber them a lot because they have had to divide their forces in order to defend both their town and the portal to the town.
Fun fact about narrow corridors, they can be defended by a very small force... That's why Thermopile was such a problem for the Persians.
And once again you're assuming the enemy has 2 forces attacking at the same time, which is a big waste of effort on their part, as they would be better off just attacking with the army that's not using the portal, and not have any of their forces at a choke point, as all the things you said they could do at the portal would work just fine against fortifications without one...
especially the earthquake spell.
And BTW, the earthquake spell doesn't actually hurt creatures in HoMM, never has...
And nothing keeps one of your mages from casting protection from fire on himself and then casting the earthquake spell on the annoying gatehouse thing. No more mages, no more firewalls.
Yeah, except stuff like arrows, any non-fire spells or a boot to the head...