What type of wall is referred to as 'Opus incertum'?

Opus incertum refers to a specific technique in Roman wall construction characterized by the use of irregularly shaped stones. These stones are typically placed in a random or irregular pattern, which are then set within a concrete core. This method allows for a strong structural integrity while also providing an aesthetically intriguing façade. The randomness of the shapes of the stones contributes to the uniqueness of each wall, distinguishing it from other more uniform methods of construction.

In ancient Roman architecture, this technique was commonly employed to create robust walls for buildings such as villas, temples, and public structures, allowing for both structural stability and a distinctive appearance. The use of irregular stones also enabled builders to utilize locally sourced materials effectively, contributing to the adaptability of Roman architecture across different regions.

The other options mentioned do not accurately describe Opus incertum; the reference to clay tiles or square bricks does not align with the characteristic irregularity of the stones used in this technique, and a wall made of stone slabs typically refers to a more uniform construction style that does not embody the random placement of materials seen in Opus incertum.