A castle buttress is a solid vertical masonry projection built against an exterior wall to absorb lateral forces, prevent structural bowing, and reinforce fortifications against siege bombardment.

Structural Form and Mechanics:
Massive Projections: Built as thick, heavy stone pillars bonded directly into the core of the curtain wall or keep.

Load Redirection: Intercepts outward lateral thrust caused by heavy stone vaulted ceilings, internal floor beams, or external explosive impacts, channeling the kinetic energy safely down into the foundation.

Pilaster Strips: Often designed in early medieval architecture (such as Norman keeps) as shallow, flat vertical wall pilasters that stiffen long stretches of flat masonry.

Defensive and Tactical Adaptations:
Bombardment Resistance: Added to vulnerable curtain walls or widened later in the medieval period (e.g., the 16th-century reinforcements at Carlisle Castle) to absorb the shockwaves of early gunpowder artillery and cannon fire.

Corner Reinforcement: Frequently styled as angled, diagonal, or clasping buttresses where two high defensive walls intersect at a 90-degree angle to prevent the corners from shearing apart under stress.

Defosading/Dead Zones: Designed with sloped offsets or set back profiles to prevent attackers from finding a clean blind spot at the base, though sharp angles could sometimes offer footholds if not carefully integrated with flanking fire.