Rath

Rath - Low, circular ringwork.
Ravelin - Outwork with two faces forming a salient angle; like in a star-shaped fort.
Rear-arch - Arch on the inner side of a wall.

Rath: An Irish earthwork fortification dating from the Iron Age.
Ravelin: A detached outwork developed from the demilune, consisting of a triangular work with two embankments raised before the counterscarp, ...

Rath(Irish): Circular or near-circular enclosure consisting of one or more earthen (or occasionally stone) banks, classified according to the number of surrounding ditches as univallate, bivallate or trivallate.

See also: Masonry, House, Hall, Shell, Moulding