Earthworks are large structures of earth that have been built for defence, especially ones which were built a very long time ago.
Europe's biggest Stone Age earthworks are at Silbury Hill in Wiltshire.
earthwork in British English
(ˈɜːθˌwɜːk)
noun
1.
excavation of earth, as in engineering construction
2.
a fortification made of earth
earthwork in American English
(ˈɜrθˌwɜrk)
noun
1.
an embankment made by piling up earth, esp. as a fortification
2. Engineering
the work of excavating or building embankments
3.
a work of art, usually massive in scale, incorporating a natural feature of an area, as a groove sculpted into a desert mesa or a wheat field planted in geometric patterns