As we shall see in Chapter 1, two building traditions existed side by side—the earth and timber structures of northern and western Europe and the masonry buildings of the Roman Empire and the Mediterranean world. Surrounded by great forests, the Celtic people in northern and western Europe turned to timber and earth for building materials. They lived in villages of timber and turf houses surrounded by ditches, embankments (earth removed for the ditch was piled beside it to make a bank), and wattle (woven branches) fences. Wherever possible they chose defensible sites. For example, the Celts in France (known as Gauls) built their villages on hilltops with commanding views and difficult access, while in Ireland they built on islands in lakes or swamps. They strengthened the sites with timber palisades and towers and complex earthworks including ditches filled with spiked poles. On a relatively secure site they might use only wattle fences that formed corrals rather than effective defensive works. These cities and forts usually had a circular plan, since a circular wall encloses the most land within the shortest wall. When the Gauls confronted Caesar (58–49 b.c.e.), they retreated behind the massive earth and timber walls of the hilltop oppidum, Alesia. The Romans then laid siege and defeated them by using complex siege engines such as battering rams and catapults.