To determine which element is constructed at intervals along the length of a wall to stabilize it against overturning, we need to understand the purpose and function of each option provided:

• Barrel Vault: A barrel vault is an architectural element formed by the extrusion of a single curve (or pair of curves, in the case of a pointed barrel vault) along a given distance. Essentially, it's a series of arches placed side by side to form a continuous vault. This element is primarily used to cover a space and does not stabilize walls against overturning.
• Pilaster: A pilaster is a shallow rectangular column that projects slightly beyond the wall into which it is built. Its primary function is decorative, and while it can support some weight, it is not designed specifically to stabilize walls against overturning.
• Squinch Arch: A squinch arch is used to transition from a square space to a dome and is commonly found in domed structures. Its purpose is not related to stabilizing walls directly but rather to provide support for domes.
• Buttress: A buttress is a structure built against or projecting from a wall which serves to support or reinforce the wall. In its most basic form, known as a 'buttress,' it is used to stabilize a wall against lateral forces, such as those caused by the vaults or wind loads, thereby preventing overturning.

The correct answer is Buttress, as a buttress is specifically designed to reinforce and stabilize a wall by counteracting the lateral forces that can cause it to overturn. This is a common architectural solution in historical structures and is frequently seen in Gothic architecture.