Roman villas in northwestern Gaul  Roman villas in northwestern Gaul (modern France) functioned as colonial economic centers. Most villas did not resemble the luxurious, aristocratic country retreats of the Mediterranean region. Their owners were absentee investors (or the emperor himself), managed by local Gauls whose families were rewarded after the Gallo-Roman wars. It is difficult for archeologists to define a villa; the recovered residences varied in size and style (often determined by economic function). However, all sites designated as "villas" contain Roman architectural elements found in homes (such as mosaics, porticos, columns and square grounds plans). At first the new Roman masters physically changed very little in Gaul, simply refining the rural economic system in an already intensely farmed landscape. These refinements took the form of technological improvements and enhancing the economic structure (which included the transport of goods and raw materials to larger markets). The Gauls (Celts who lived in Gaul) were a culture rather than a race, nation or empire. They were skilled in metalworking and cattle-raising. The culture began to dominate France around 800 BCE, replacing the existing culture (but not the people). The Celtic landscape resembled a countryside, with open fields instead of woods; however, Celtic fields were smaller (often square). More fields were used for pasture than for crops because of the need for cattle, sheep and forage. The Gauls intensely managed the forest for wood and forest products; the Romans enhanced the system without dramatically altering it. Villages and hamlets were denser in the countryside during the Roman period; the population (equal to that at the time of Louis XIV) was about 20 million. At this time, Roman Britain's population of four to six million equaled later-medieval numbers. Homes were better-constructed than most houses built during the Middle Ages; they were built with local materials: timber uprights with plaited wicker, coated with clay, straw and animal hair. Julius Caesar and his Roman legions succeeded in their conquest of Gaul from 57 to 52 BCE at the invitation of Gauls in Marseille, who asked the Romans to come to their defense. The Romans occupied and managed their empire with a minimum of administrators, no police force and an army primarily located on the frontiers (much like the French colonial empire of the 17th and 18th centuries. A few Roman natives lived in major towns, but most governmental and economic functions were carried out by local Gauls. This "minimalist empire" left most people and institutions untouched.