Sure, retaining walls look like simple stacked stone, block, or timber. 
But in fact, they're carefully engineered systems that wage an ongoing battle with gravity. 

They restrain tons of saturated soil that would otherwise slump and slide away from a foundation or damage the surrounding landscape. 

These handsome barriers also make inviting spots to sit, and can increase usable yard space by terracing sloped properties, something that is increasingly important as flat home sites become ever more scarce in many regions.