What architectural innovation is credited to the Markets of Trajan?

The Markets of Trajan are indeed recognized for their innovative use of concrete, which marked a significant advancement in Roman architecture. This architectural innovation allowed for the creation of vast and complex spaces that were previously unattainable with traditional building materials. The use of concrete enabled the builders to design the markets with an unprecedented level of spatial organization, including the incorporation of vaulted ceilings and broad arches that helped distribute weight effectively across the structure.

Concrete's versatility and strength facilitated the construction of multi-story buildings and expansive interiors, drastically changing the dynamics of urban architecture. This innovation was pivotal in enabling the Romans to create not just functional spaces, but also architecturally significant forms that emphasized their engineering prowess.

While the Markets of Trajan do feature multi-level structures and other elements, the defining and groundbreaking aspect of their design lies primarily in the effective integration and use of concrete as a fundamental building material.