In medieval military architecture, a chemise wall (derived from the French word for "shirt") is a low, tight defensive wall that closely encircles the base of a castle's central keep or tower. Also referred to in English architecture as an apron wall or mantlet wall, its primary function is to wrap around the most critical stronghold of the fortress like a protective garment, acting as a secondary line of buffer defense.

Primary Purpose and Strategy:
Sapping Prevention: Attackers frequently tried to collapse castle keeps by "sapping"—digging tunnels under the foundations to make the heavy stone structure cave in. A chemise wall forced enemy miners to dig much further back, away from the keep's actual base.

Battering Ram Buffer: It absorbed the direct physical impact of battering rams, protecting the vulnerable main walls of the keep from catastrophic structural damage.

Deflecting Projectiles: Many chemise walls featured a sloped base (batter), causing heavy stones or catapult missiles dropped or fired from above to bounce outward into the oncoming ranks of the enemy.

Architectural Layout and Design:
Close Proximity: Unlike a broad curtain wall that encloses a spacious courtyard (bailey), a chemise sits incredibly close to the central tower, creating a narrow, highly defensible perimeter path.

Geometric Forms: They were often circular or built using a series of interlinked, overlapping semicircular bastions to eliminate blind spots and deflect incoming ballistic fire.

Integrated Fortifications: Depending on the size of the castle, a chemise could be outfitted with its own mini-gatehouse, drawbridge, or arrow loops to reinforce its standalone defensive capabilities.