Major spoilers from Game of Thrones Season 7
Would the Night King have been able to get past the Wall without
the help of an undead dragon? It seems like there were no indications he had any other method of bringing the Wall down.
If so, doesn't that mean the southern realm would still be (relatively) safe if
Jon hadn't gone on that capture mission that required a dragon rescue?
Look, there are a few things to consider here:
The Wall was built approximately 8,000 years ago (if the legends are to be believed). It was built presumably to keep the Free Folk (known to Westerosi as 'Wildlings') out, but in reality was actually built to keep the Others (White Walkers) out. It was built by Brandon the Builder, with the help of the Children of the Forest, and seems to have magical properties.
In the show, I don't believe it is confirmed that the Wall has magical wards, but it definitely is in the books. In fact, in the scene where Sam helps Bran and co. to pass through the Wall, you can clearly see that there's no mention of the magical wards. Whereas, in the corresponding chapter from the books, Bran and his group actually need to pass through the 'Black Gate' below the Nightfort castle, and only a member of the Night's Watch can allow them to do so.
So this establishes that there is most likely some magical ward stopping the dead from passing through the Wall. It's important to mention that this same ward is believe to have been in place at the Cave of the Three-eyed Raven.
Ok, so, now we move onto the abilities of the Night King, one of which seems to be that he can 'Mark' someone. This grants him the ability to know where the person marked is, and can get past the magical wards protecting them. As proven in the episode Hold the Door.
All the Night King would have to do is wait until he can 'mark' someone able to pass the Wall, and literally follow them in. Yes, in this particular case it does help to have undead giants able to smash through the physical barriers.
Listen, all of this stuff is somehow connected, either by the fact that they are originating from the same source of magic, or that one way or another they can sense and affect each other.
It has been widely mentioned that the dragons have returned with magic, or is it the other way around? The Others seem to have returned with magic, or is that the other way around? It doesn't matter, which is my point: they are connected.
It wouldn't be too far-fetched to surmise that perhaps the Night King can feel the presence of dragons. All he'd have to do in that case is wait until one comes flying straight to him. Oh wait, he did do that!
Lol, like forreal though. There exists (at least in legend) a horn believed to have the ability to bring down the Wall, the Horn of Joramun (aka the 'Horn of Winter'). Perhaps the Night King has, or can, or would find this horn.
The Night King is a resourceful dude, he has shown capability beyond imagination. He has waited for over 8,000 years. Something tells me that he's ready and there won't be anything to stop his progress.
Unknown.
It is possible he could have gone south if the same enchantment that blocked him from entering the Cave of the Three Eyed Raven also stopped him from going past the Wall. When he broke the cave's enchantment by touching Bran, he might have also broken the Wall's enchantments.
The Night King might have been able to go under the Wall where there were openings to other Night's Watch castles.
Maybe we will get an answer when G R R Martin publishes the next book.
The show never really introduced the Horn of Joramun/Winter concept, so the Night King presumably has some plan to get south of the wall, which was quickly adapted when he acquires
a dragon
(unless he knew this was going to happen all along and was just getting ready).
It is suggested in the books (and I believe Ygritte hinted at it in the show) that it was Mance Rayders' tomb pillaging looking for the Horn that raised the ire of the Others and started off their slow invasion.
“We never found the Horn of Winter. We opened half a hundred graves and let all those shades loose in the world, and never found the Horn of Joramun to bring this cold thing down!” - Ygritte to Jon Snow in Storm of Swords, chapter 30
Perhaps their end game in the show was just to drive humans south of the wall, but once they got a dragon they decided to keep on going for Russia :)
I suspect the show writers used the dragon as a substitute for the Horn. But can the Night King (or the Others in the books) get past the wall in other ways? I suspect they can. We have seen that wights are not simply destroyed when crossing the wall, so presumably the wall magic (if any actually exists) is a passive barrier, not an active one that banishes the animation magic or destroys the undead. The magic just keeps the ice wall from crumbling under its own weight and Coldhands/Benjen BELEIVES it keeps him from passing (he was indoctrinated in all of the Night's Watch teachings, after all), but has never actually tried it. Perhaps the wall COULD be climbed by the undead, or simply undermined or slowly tunneled through, hence the creation of an order of men to safeguard it. If the wall was just perpetual magic and invulnerable, there would be no need for the Night's Watch. The Night's Watch, at least as far as we know, was created to defend against the White Walkers, not wildlings, so presumably the white walkers pose a threat to the wall itself so it has to be maintained and defended lest it fall to ruin or be breached.
