Long Cliffs are the cliffs near the Abhorsen's House. The Old North Road passes on top of them. There is a tunnel to the side near the Abhorsen's House which is protected by Charter sending.
In Sabriel she uses the tunnel to get away from the Mordicant sent by Kerrigor to get to the house. The sending guarding the tunnel helps her by keeping the Mordicant away for a time.