Britain during the reign of Julius Caesar had an Iron Age culture, with an estimated population of between one and four million. Archaeological research shows that its economy was broadly divided into lowland and highland zones. In the lowland southeast, large areas of fertile soil made possible extensive arable farming, and communication developed along trackways, such as the Icknield Way, the Pilgrims' Way and the Jurassic Way, and navigable rivers such as the Thames. In the highlands, north of the line between Gloucester and Lincoln, arable land was available only in isolated pockets, so pastoralism, supported by garden cultivation, was more common than settled farming, and communication was more difficult. Settlements were generally built on high ground and fortified, but in the southeast, "oppida" had begun to be established on lower ground, often at river crossings, suggesting that trade was becoming more important. Commercial contact between Britain and the continent had increased since the Roman conquest of Transalpine Gaul in 124 BC, and Italian wine was being imported via the Armorican peninsula, much of it arriving at Hengistbury Head in Dorset.