When the Roman Empire began its expansion through Mid and Western Europe it was met with very fierce resistance from the local tribes, called the Celts. The Celts had dominated this area for over a thousand years prior to the Romans. Perhaps originally from the lands later known as Germanic, they had, between 500 BCE. and 100 BCE, spread outwards in migratory waves that took them eventually from Ireland and Spain to Turkey in their quest for newer lands and better opportunities. Creating a new life for themselves involved dislodging the original inhabitants, who naturally weren't exactly welcoming, and so habitual warfare became a way of life with the Celts, establishing their reputation permanently as a quarrelsome, war-mongering lot. Their invasions however were periodic irritations rather than large concentrated attacks. For the Celts, although they were people with a common ancestry, language, and religion, were deeply divided into many different tribes, each loyal to their own Chieftain and having their own social structure and customs, including coinage. More often than not these tribes were at war with one another as well, and they actually seemed to relish it too.
According to the accounts left by Greek and Roman writers of the period, wars were always elaborate affairs, with finely embellished weapons, armors and chariots brought into use; although many of the warriors were also known to eschew everything and rush headlong and stark-naked into battle. Their courage was legendary and most of them were very tall and sturdy, with wild, bleached-blond hair and enormous mustaches. They usually heightened their daunting appearances with blue woad dye and horned bronze helmets, and then further unnerved the enemy before battle by issuing weird, spine-chilling screams while clanging up a fearsome cadence with their swords against their shields. Working themselves into a terrible fury in this way and disheartening the enemy, they charged into battle and fought with incredible savagery.
Their preferred mode of fighting was using guerilla tactics rather than straight head-on. They made swift, lightning raids in small groups of foot soldiers, cavalry and chariots, attacking adeptly with bows and arrows, swords, spears, javelins, and slingshots, harassing the enemy from an unexpected side and then withdrawing before the enemy could fully retaliate while another group attacked from another side. These unusual methods of fighting quite annoyed the disciplined, orderly Romans, who, often put to loss, dubbed them as cowardly. They certainly were quite disconcerted too by the Celtic penchant for chopping off enemy heads and hanging them from their belts or around the necks of their horses to be later on proudly displayed as mounted trophies atop door-posts or embalmed with cedar oil in special boxes. The head-taking was not only supposed to endow the Celtic warrior with the additional powers of his fallen foe, but the greater the number of displayed heads the greater was his social standing. The Celts were a rough crowd, and had some pretty primitive and blood-thirsty customs. Another one of these apparently involved ritualistic animal and human sacrifices, carried out in woods and near watering places by the infamous Druids. There is, of course, some controversy regarding this since the Roman view-point which we have is not exactly an unbiased one. They were not likely to be glorifying their enemy and it suited their purposes to show the Celts as barbaric and immoral as compared to their own civilized ways.
The Druids were the Celtic Priests and particularly abhorred by the Romans. Not just for their grisly religious practices, but also for the enormous influence they wielded within their Tribe, usually greater than even that of the Tribal Chieftain and always to Roman detriment. The Druids were not just priests, but political advisers, healers, teachers and mediators. As the Celts leaned more towards oral tradition rather than a written language, the passing on of information and history and customs was done by word of mouth, usually sung in poetic format and fine Celtic timbre, and this was the responsibility of the Druids as well as the bards and poets. Celtic history, often inseparable from mythology, full of accounts of fairies, dragons and so on, and these tales were diligently saved through troubled times. The Druids were also responsible for keeping the Celtic calendar, which, according to Caesar and Pliny, was counted by nights and not days as the Celtic belief was that the earth came into being in darkness and it was later that the light came. Similarly they believed that winter (beginning November 1) preceded summer (beginning May 1), and their various festivals, commemorating the large pantheon of Gods, were celebrated in accordance.
The Celtic Clan was the life-blood of the Celts and to be excommunicated from it was the ultimate disgrace. Headed by the Chieftain, it was categorized into the Druids, the warriors, and the commoners. Families usually lived together and, as polygamy prevailed, these were quite complex and large. The polygamy practice didn't however adversely affect the social standing of the celtic women, which was far better as compared to the other contemporary European societies. In many ways they had equal rights as the men, could own and inherit property, and could marry according to their own choice. In some cases they were also able to assume charge of their tribe and didn't do too badly either, as is seen in the example of the Icenic Queen Boudicca. The children were jointly reared, often fostered out with near relatives, and this deepened the family ties further. It was considered the duty of the family, from both paternal and maternal sides, to take up on behalf of a wronged or insulted member and so personal feuds often enough transcended into tribal feuds.
The Celts normally lived in small hamlets comprising several roundhouses and encircled by protective stone or wooden palisades. The roundhouses were made of arched timbers with either wattle and daub or stone walls and thatched roofs of straw or heather. Very often, since the Celts were perpetually at war, hill forts for stronger defense purposes were built. Strategically located and protected additionally by tall palisades and deep ditches, these ranged from small family-sized affairs to somewhat bigger ones for a larger number of people. As there were no water sources inside these hide-outs, it is assumed that they were utilized only as short term shelters during enemy attacks. In these situations, the Celts would also take their sheep and cattle with them. These herds were very precious and the person having the largest number was the wealthiest. The tribal lands however seemed to have been communally owned. When the Celts were not at war, they were farmers, herders, iron workers, and artists. They were the first people to bring iron working to the British Isles, where iron was found in plentiful quantities. With this knowledge, particularly with their iron plow, farming became easier and more lands could be brought under cultivation. The artistic side of the Celts was displayed not just in their poetry and music, but also in their metal-work, pottery, textiles, tattoos and later on, most famously, in their illuminated manuscripts. This art was characterized by a pleasing combination of fanciful, curvilinear motifs and intricate, geometrical designs. The Celts also had towns or 'Oppida' as they were called and these don't seem to have been permanent settlements either, but gathering places for annual festivals and trading.
Some of the main Celtic Tribes in Europe were the Aedui (France), the Helvetii (Switzerland), the Boii (Italy), the Eravisci (Hungary), the Volcae (Anatolia), the Brigante (Northern England), the Parisii (Paris), the Nervii (Belgium), the Venetii (Armorica), the Arverni (Auvergne), the Scordisci (Beograde), the Durotriges (Dorset), the Iceni (Norfolk and Suffolk), and the Trinovantes (Eastern England, north of the Thames).
All of them were eventually either defeated and annihilated by the Romans or, with force or diplomacy, made their feudatory's. The reason for the final Roman victory lay both in their shrewd politics and well-disciplined army as well as the traditional feuds and inherent disunity amongst the Celts.