A sizeable proportion of the images in this archive come from Rome, where there is the largest concentration of sculpted monuments anywhere in the Roman world, as well as many of the most important collections of Roman material. Most of the ancient monuments still standing in Rome date from the Roman imperial period, particularly the 1st-4th centuries AD. Rome was the artistic and architectural centre of its empire, as well as the commercial and political one. It drew on the extensive resources of all of its territory and this is clearly revealed by the huge quantities of imported stone used in its monuments. The local tuff and travertine found around Rome, while suitable for basic construction, is of limited use for fine carving and from the 2nd century BC onwards Roman architects and artists increasingly imported marble from the Aegean. In the 1st century BC polychrome materials began to imported from North Africa and Asia Minor, and by the 1st century AD decorative stones from Egypt were also exploited. Many of the architects and artists operating in Rome, especially in the Republican period were themselves Greeks and the city remained a hub of immigration, a fact which left a major impact on its artistic and architectural landscape.