Juniper is known to last 30+ years in ground-contact installations, without the need for additional  chemical treatment, and has a beautiful, natural look to it that other types of wood simply cannot offer. 

Here are some popular styles of juniper retaining walls and the specific lumber dimensions that were used to create them:

Juniper 2x6 stacked and supported with 4x4 vertical posts
(photo credit Inner City Farm)

Stacked 6x6 juniper timbers

Stacked 5x5 juniper timbers paired with decomposed granite

Juniper supported with iron anchors weathers to a silver gray over time and adds natural beauty at the campus of University of Washington, Tacoma