Ever Been to Scotland, or any Other Rocky Place?
What did they do with all those rocks..? Well Jeju is a Volcanic Island, and full of stone, just like Hawai'i, and just as the ancient Hawaiians used the ubiquitous basalt to build EVRYTHING, so too did the ancient Jeju Islanders. You've already seen picture sof the stone houses. Now you see the rocks used as rocks walls.
These walls often demarcate not only houses and their Madang or Courtyards, they are often also used to shelter paths (Walls of both sides) jeju is somewhat windy you see. But most commonly you will see these walls demarcating boundaries and providing shelter as shelter belts for non wind hardy crops.
All the same. they are beautiful huh.

The wood of the yew tree
Ben on Udo Island