The end of the Iron Age extends into the very early Roman Empire under the theory that Romanisation required some time to take effect. In parts of Britain that were not Romanised, such as Scotland, the period is extended a little longer, say to the 5th century. The geographer closest to AD 100 is perhaps Ptolemy. Pliny and Strabo are a bit older and therefore a bit more contemporary, but Ptolemy gives the most detail and the least theory. Archaeological evidence Attempts to understand the human behaviour of the period have traditionally focused on the geographic position of the islands and their landscape, along with the channels of influence coming from Continental Europe. During the later Bronze Age, there are indications of new ideas influencing land use and settlement. Extensive field systems, now called Celtic fields, were being set out, and settlements were becoming more permanent and focused on better exploitation of the land. The central organisation to undertake that had been present since the Neolithic period but became targeted at economic and social goals, such as taming the landscape, rather than the building of large ceremonial structures like Stonehenge. Long ditches, some many miles in length, were dug with enclosures placed at their ends. Those are thought to indicate territorial borders and a desire to increase control over wide areas. By the 8th century BC, there is increasing evidence of Great Britain becoming closely tied to Continental Europe, especially in Southern and Eastern Britain. New weapon types appeared with clear parallels to those on the Continent, such as the Carp's tongue sword, complex examples of which are found all over Atlantic Europe. Phoenician traders probably began visiting Great Britain in search of minerals around this time and brought with them goods from the Mediterranean. At the same time, Northern European artefact types reached Eastern Great Britain in large quantities from across the North Sea. Defensive structures dating from this time are often impressive such as the brochs of Northern Scotland and the hill forts that dotted the rest of the islands. Some of the most well-known hill forts include Maiden Castle, Dorset; Cadbury Castle, Somerset; and Danebury, Hampshire. Hill forts first appeared in Wessex in the Late Bronze Age but became common only in the period between 550 and 400 BC. The earliest were of a simple univallate form and often connected with earlier enclosures attached to the long