A wardstone was the anchor of the magical defences of a big property, as protection spells could scarcely keep hold over a large swathe of land without being dispersed over time. Though it was far from the only way to make magical defences hold- Jon had thought that runic inscriptions were carved in the foundations of the Winterfell's walls instead, anchoring the magical protections differently. Or mayhaps made permanent with some sort of blood magic or sacrifice.