In castle architecture and medieval warfare, a mantlet (or mantelet) refers to two distinct defensive features: a stationary architectural secondary wall built into a castle's design, and a mobile wooden shield used primarily by attackers.

The Architectural Mantlet Wall (Zwingermauer). In permanent fortifications, a mantlet wall is a subsidiary, low outer wall constructed closely around a vulnerable building or a primary defensive line. Its main purpose is to act as a physical buffer and create a lethal bottleneck for intruders.

The Killing Ground (Zwinger): The space between the main castle wall and the outer mantlet wall forms a narrow, roofless enclosure. If attackers scale or breach the mantlet wall, they find themselves trapped in a tightly confined courtyard directly exposed to flanking fire from the much higher main castle towers above.

Keep Protection: Mantlet walls were frequently wrapped tightly around a circular keep or the main gates. They absorbed the initial shock of battering rams, catapults, and early artillery, shielding the foundations of the primary keep from direct siege damage.

Defensive Features: Though lower than the main curtain wall, architectural mantlet walls were heavily fortified with their own battlements, arrow slits, and later, musket or gun loops.

Tactical Usage in Sieges:
Advancement: Provided mobile cover for archers, crossbowmen, and sappers (moat-diggers or wall-breakers) to move within effective range of a castle.

Assault Support: Served as a frontline barrier or stepping point during operations to scale or breach enemy fortifications.