Consider that is where we arrive when first traveling there, it is easy to assume that the High Wall simply refers to Lothric’s outer defenses. The walls we warp atop do tower high above the city, with only the castle at its center surpassing them in height. But the description for the Small Lothric Banner makes a point of this High Wall arising and breaking the road to the Undead Settlement. Once we open the massive main gate, we can spy remnants of a broken road out of the city, a giant bridge to the Undead town nearby — far below and in shambles. With the rest of the outside word just as low, we can conclude that the “High Wall” refers not to just the perimeter defenses but also the massive mountain they sit upon. The mountain likely always existed if a great bridge was supporting the main road in and out of the kingdom, but it evidently was never this tall. Something caused it to suddenly rise up.
Indeed, close inspection of the High Wall area reveals parts of the city within seemingly torn asunder, gaping holes to deep chasms interspersed among the buildings. In some cases, we even see infrastructure without a base, though this is probably just be a product of lazy map design; various other areas in the game suffer from similar problems without such an exculpatory explanation handy. In the case of the Cemetery of Ash, the area looks to be at an unnatural elevation, fragmented, but looking at the adjacent Lothric walls, we can see infrastructure stretching down to the bottom of the supposed High Wall — in other words, parts that should have originally been underground. There are definitely inconsistencies in this concept’s portrayal. Regardless, we can see similar fissures in the earth in the Undead Settlement also, so we can still safely presume that the immediate region around Lothric has suffered from significant seismic activity. The only question is why.
Given that the kingdom is already the focal point for the drifting lands, sporadic seismic activity of this nature wouldn’t be too far removed. In that case, the cause is likely still the First Flame located somewhere below the city. But if the High Wall is another byproduct of the world’s stagnation, why has it seemingly only appeared recently when the lands have been drifting since before Lothric’s foundation? The most likely answer is because Prince Lothric refused to link the fire. This decision has already resulted in a similar radical reaction from the pus of man, and the High Wall emerges around this same time of the First Flame reaching the emergency threshold. Taken together, the High Wall was probably the first step in the various lands breaking apart and forming the Dreg Heap, only kept from progressing further thanks to Firelink Shrine awakening the Lords of Cinder. But with the eclipse manifesting after their deaths, Lothric’s inevitable future will finally come to pass.