A retaining wall is a structure designed to hold back soil and prevent erosion on slopes or where grade changes create level differences. Retaining walls allow development on sloped sites, protect infrastructure, and create usable flat areas. They're constructed from various materials including concrete, stone, timber, and engineered block systems. Municipal building codes regulate retaining walls, with permits typically required above certain heights. Improper retaining walls can fail, causing property damage and safety hazards. Engineers design larger retaining walls to handle soil pressure and drainage. Municipalities build retaining walls along roads, in parks, and to protect public infrastructure on slopes.