A berm in castle architecture is the flat, level strip of ground located between the base of a main defensive wall (curtain wall) and the inner edge of a protective ditch or moat.

Why Berms Were Used:
Medieval and military engineers incorporated berms into fortress layouts for several specific strategic and structural reasons:

Structural Stability: The berm prevented the heavy weight of stone walls and ramparts from pushing the soil into the ditch, keeping the earthwork stable and preventing the wall's foundation from collapsing into the moat.

Preventing Undermining: By setting the wall back from the edge of the ditch, it made it physically harder for enemy sappers to dig tunnels directly underneath the wall to collapse it.

Defensive Chokepoint: If attackers managed to fight their way down into the ditch and scale the opposite slope, they were met by defenders standing on the berm outside the main walls, creating a highly effective kill zone.

Variations:
Width: The size of a berm could vary drastically depending on the age and style of the castle. Many early earthwork or Motte-and-Bailey fortifications featured wider berms, while later, sophisticated stone castles often narrowed them or eliminated them entirely to allow the walls to rise straight up from the water's edge.

Material: While typically made of compacted, leveled dirt, berms were sometimes paved or reinforced with stone to make them sturdier and to prevent them from becoming muddy or eroded by weather.