Any well-fortified structure located on a hilltop and enclosed by at least one wall of stone and earth, commonly referring to sites of the Late Bronze Age or Iron Age. The earliest date to c 1000 BC. Some hillforts contain houses and were perhaps royal residences or, in the case of large forts of oppidum type, true towns; others seem to lack permanent buildings, and were probably refuges where the people and flocks from the surrounding area took shelter in times of crisis. At first they were usually promontory forts, but in the last four centuries BC the true hillfort, with defense works following the contours, became the predominant form. From about the second century BC until the Roman conquest, hillforts were common throughout Celtic lands. In Britain most of the great forts were built during the two and a half centuries before the conquest of 43 AD, but in Ireland and highland Britain hillforts continued to be built and used for several more centuries. They are found throughout much of Europe, except Russia and Scandinavia. In size, hillforts ranged from less than one acre to several hundred acres.