A curtain wall is the defensive wall surrounding the bailey of a medieval castle. It can also be a defensive wall between two bastions of a castle or fortress in post-medieval fortifications.

In earlier designs of castles the curtain walls were often built to a considerable height and were fronted by a ditch or moat to make assault difficult.

Later with the introduction of trace italienne fortifications the height of the curtain walls was reduced and beyond the ditch additional outworks such as ravelins and tenailles were added to protect the curtain walls from direct cannonading.