ring wall
/ˈrɪŋ wɔːl/noun
1A wall completely surrounding or encircling a certain area.
2A circular wall forming the refractory lining of a furnace or kiln. Now historical.
3Now chiefly Science Fiction. A roughly circular ring of cliffs or mountains enclosing an area such as a crater or a lunar mare.
Origin
Early 19th century; earliest use found in Julius Hare (1795–1855), author and Church of England clergyman. From ring + wall, probably after German Ringmauer.