Vanya and the Green Knight
Every righ sends people to ride it per year, each riding lasts for a season, a holdover from when Armagh was united under the Ard Righ.
And it is every ruler, human or elf or dwarf, sends a group of his people, usually warriors, to help defend and patrol the land, called ‘Sent to the Wall’. The rulers set this up in different ways. Maybe they have a mage who scries and would use their own army if something came up. Maybe they have a band of mounted warriors who are permanently on patrol far and wide of their lands. Maybe its a coming of age you have to do it for this long, like the Israeli military.
Three countries directly border the Wall- Albion, Caerllew, and Armagh, but it is only once one gets to Armagh that the wall rises from a border marking to a real barrier, high smooth and white, blocking access from the canal.