Walls and Barriers

There are tons of walls in this story, the most obvious of which being the capital-W one that divides the Old Kingdom and Ancelstierre. This Wall is a crucial barrier, serving to separate the magical and non-magical world. Soldiers patrol the perimeter, armed with both modern and medieval weapons, and it's defended by "[…] barbed wire, bullets, hand grenades and mortar bombs" (2.2). All this suggests that it's pretty important to keep magic and technology separate in this world.

We've also got the walled-in, fortress-like Abhorsen's House, the walls and aqueducts surrounding the capital city, and the giant boom chain stretching across the mouth of the sea at Belisaere—yep, there's even a barrier hanging over the bay near the capital.

Metaphorically speaking, there's the invisible barrier between life and death, which necromancers get to know pretty well, and every time we see a wall or other sort of barrier, we are clued into the fact that we're in a place where life and death are dangerously close to one another.

All of these walls and barriers serve another really important role: they keep things where they should be. Dead things should stay on the Dead side; living things should stay in Life. It's risky to cross any wall in Sabriel's world. Just as Colonel Horyse and his troops devote themselves to patrolling the Wall, Sabriel and the other Abhorsen risk their lives to make sure that nothing gets out of Death that shouldn't be in Life. And in this way, walls and the like also symbolize order. There's a system in place here that should work so long as everything stays where it belongs.