We didn't really spend the entire design phase doing things about walls, the Wall-Breaker can only make one hole at a time and we (very obviously, to anyone who knows anything about sieges) need to attack multiple places at once if we want to, y'know, not die to the massed defenders, and our spies are there to make their forces generally less capable. I don't expect them to succeed so incredibly as to actually open a gate for us.
And the Wall-Breaker will not be able to just keep bashing holes, because he exists for a reason that is not encapsulated in his name. Specifically, I wrote a Champion literally made of brute force and just-won't-die because he's meant to tie down any and all Heroes that attack us right now. His name is just a cooler title than "Meat Shield With Hammer". We won't always be facing walls but we will, from the sound of it, almost always be fighting Heroes, and somebody who is capable of preventing them from doing anything other than facing him for a really long time is super, super useful.
And none of this still actually deals with the fact that ranged combat is important, scouting is important, even in a siege battle, ESPECIALLY when the enemy is literally divinely guaranteed to get reinforcements, and our present forces are not up to the task for it. Once we're in the streets, heavy infantry trumps cavalry. But if something goes wrong (and please for the love of all that is good evil morally neutral and interested in crafting and making the Orcs into a real civilization remember that this game is built around literal divine mandates about things going wrong for us) we might end up facing their forces on the field, with our Champion tied down doing other things.
Actually we might be able to get the Shadow-Walker to actually get a gate open/prevent one from getting closed when we begin our attack...but that's something for the strategy phase.