Honestly probably focusing on the south gate was a mistake given how they don't seem to be moving outside their main area (except maybe goblins), I wouldn't mind moving the foundry actions to other pursuits. I'm not sure about ladder poles though, given that we don't really have a good idea of how they are going to attack I'm afraid that we could end up making ladder poles and then they never use ladders to attack.
As for spearmen vs archers. Well, I'm just not sure about how combat works to make a real decision there. I thought archers sounded good because they fight as good as spearmen during combat, but get a free attack at the start, presumably making them as good as veterans in the first round of combat. So it really depends on how long we can expect combat to go. If combat is just about a round, archers are better, and they are fairly equal if combat goes for two rounds (archers are better then veterans during round two but worse then spearmen, but both pretty minor differences) veterans become better during the third round of combat. But if archers are killing enemy units even before combat starts, we'll fall into lanchester's law and archers will be better for much longer then they originally appear. Archers also have the side benefits of being able to turn into marksmen and to cover walls one section away instead of veterans presumable zero sections away, But without any idea of how combat works it's impossible to make the determination of what's better and if those additional benefits are worth anything. For all we know units could have different levels of durability that completely changes these calculations or something.
Also apparently some enemy units are ranged. No idea how good they are, but maybe they might have enough to like, try to shoot off our defenders, in which case we'd need archers to defend against such attacks.
As for them getting directly into melee during night, Kashyyk said they can only do that if they don't do anything else including things like using ladders. I don't know if there's a way they can actually get on our walls without us noticing. Like maybe some goblins can climb walls? But, once again it's more hidden information but it seems like they probably can't do that if we have intact walls.