The Celts:

History: the Celtic culture and language came into existence around 1200BC and probably originated from the Indo-European Bronze Age cultures of central Europe and the Balkans, the cradle of this early Celtic culture is believed to have been in western Rumania and Hungary.
During the 19th century the Celts were often considered to be a homogenous people with common origins but modern historians have proven this theory to be incorrect, the Celtic identity only consisted of a common culture and language that was adopted by a wide variety of peoples throughout Europe and may have been a mixture of various native cultures that were held together by their Indo-European similarities.
The Celtic identity was quickly spread over Europe and its cultural center became Austria and southwestern Germany where eventually a rich iron culture evolved that is now called the "Hallstatt culture", this culture had been influenced by the Urnfield culture and also adopted various aspects from neighbouring peoples, after the decline of the Hallstatt culture a new Celtic culture came into existence in Gaul (France), which is now called the "La Tčne culture".
For a long time the Celts were the most powerful people in Europe and their territories included Ireland (Eriu), Great Britain (Albion), France (Gallia), Spain and Portugal (Celtiberia), Northern Italy (Gallia Cisalpina), Eastern Europe, the Balkans, and even parts of Turkey (Galatia).
The power of the Celts was so big that even the Romans had to pay tribute to them, on July 18, 390BC a Celtic army under Brennus invaded Italy and besieged Rome, eventually the Romans were forced to pay a large amount of gold to save their city from plunder and they started to weigh their gold to define the amount they had to pay, but the weights used by the Celts were too heavy and the Romans had to pay much more than was agreed, when the Romans complained about this Brennus threw his sword onto the balance to increase the weight even more and said; "Vae Victis!" (Woe to the conquered!).
Eventually the power of the Celts started to fade due to the pressure of other expanding peoples like the Romans, Germans, and Dacians; in southern Germany the Celts were defeated by the Germans and northern Gaul (Germania inferior) was settled by Germanic tribes like the Tungrians, Nervians, and Suebians, the eastern territories in the Balkans were slowly absorbed by the Dacians and the Romans conquered northern Italy (Gallia Cisalpina) and southern Gaul (Gallia Narbonensis).
In 58BC a Roman army under Julius Caesar invaded Gaul and defeated many Celtic tribes in a military campaign that is known as "the Gallic wars", in 52BC the last resisting Celts in mainland Europe were defeated by the Romans at Alesia and their famous leader Vercingetorix was brought to Rome and ritually strangled during the victory parade.

The Celts on the British isles were left alone for some time but in 43AD the Romans also decided to plan an invasion there, the Celts in Great-Britain (especially the Britons) had a culture that differed very much from the mainland Celts and many historians believe that it is incorrect to refer to them with the word "Celts" because although they spoke a Celtic language they had an almost unique culture that had less in common with the Celtic culture than has previously been assumed, the tribe of the Picts in the Scottish Highlands are even believed to have spoken a non-Celtic language, possibly even a non-Indo-European one, the Celtic identity of the British isles is believed to have been the strongest in Wales and Ireland.
In 43AD the Romans invaded southern Britain and after a series of battles the Britons were defeated and subjected to Roman rule, but they remained rebellious and gave the Romans a very hard time, some years later the Romans decided to end this constant rebellions and tax-dodgings by setting an example; they captured queen Boudicca from the tribe of the Icenians and tortured her in public while her two daughters were raped by Roman soldiers, as you can imagine the British tribes were outraged by this and they all joined queen Boudicca, who started a rebellion against the Romans and attacked Camulodunum Colonia (Colchester) with an army of 100,000 men and slaughtered its inhabitants, the remnants of the city were put to the torch.
The Romans reacted by sending the IXth Legion Hispana but this legion was defeated and Boudicca marched on towards Londinium (London), but the Romans had recovered themselves and after some heavy fighting the army of Boudicca was defeated, Boudicca survived the battle but later poisoned herself so that she did not had to live in slavery, her rebellion had failed.
In the years therafter there were many more rebellions but the Romans managed to end most of them, often in such brutal ways that entire lands were laid to waste, Calgacus, the leader of the Caledonians even said; "Ubi solitudinem faciunt pacem appellant" (They create a desolation and call it peace).
Eventually most of southern Britain was subjected by the Romans and during the period of the Great Migrations England (and later also Scotland) was settled by Germanic tribes like the Anglo-Saxons and the Norse, only in the more remote areas of Europe the Celtic culture managed to survive, the descendants of the Celts can nowadays be found in Bretagne, Cornwall, Wales, the Scottish Highlands, the isle of Man, and Ireland, other traces of the Celtic culture can be found in northern Spain and central France.

Culture: in most areas the Celtic culture was mixed with local customs which created many variations of it, there were also many different subgroups within the Celtic culture, the most important ones were the Gauls, the Picts, the Scoti, the Britons (the Picts, Scoti, and Britons are not always counted among the Celts), the Irish (and their descendants in the Scottish Highlands and Wales), and the Celtiberians (who lived in Spain and were a mixture of native Iberians and Celtic invaders).
There were also many other smaller cultural subgroups like for instance the Celts on the Balkans and the Galatians who inhabited Galatia in modern Turkey in 278AD, there have even been some small Celtic groups in the Ukraine.
There are also some sources that claim a Celtic presence in Denmark and the upper-Rhine area of the Netherlands but those are incorrect, this theory is mostly based on Celtic findings like the Gundestrup cauldron but does not take into consideration that Celtic objects may have ended up in Germanic lands as spoils of war or via trade.

Scene from the movie Braveheart In contradiction to the Germans the Celts were a bit less democratic; they were ruled by kings, nobles, and druids who often had almost absolute power, though the opinion of the population was often also taken into consideration.
The Celts were originally very powerful and considered themselves to be the rulers of Europe, this is also reflected in their names; Celtae ("The Exalted Ones"), Belgae ("The Proud Ones"), and Brigantes ("The High Ones"), the Celts also loved visual display and grandeur which is reflected by their beautiful art and their way of fighting; during battles they wore colourful cloathing with feathers, valuable materials, and other decorations.
The Celts also brought human sacrifices (something that was common in ancient Europe) and performed headhunting, they believed that the soul resided in the head so by taking that head one could posess the soul of the warrior they had defeated, they dried the heads of their enemies and hung them on their houses or their belts, French archeologists have even found some sort of temple that was made from headless human skeletons that were positioned upright with their battle equipment next to them.

During battle the Celts did not really use any formations; they simply massed together in a warband and started shouting, screaming, and taunting at the enemy to demoralize them, there are even stories of Gauls who showed their butts to the Romans (a good example is given in the movie "Braveheart" where the Highlanders show the English where they can stick their plans of ruling Scotland), the Celts also used a type of horn called "Carynx" to frighten their enemies, after they were tired of insulting their opponents they attacked; they were often victorious due to their large numbers and the individual strength and skill of their warriors, though their weak spot was their lack of tactics and their inability to adapt themselves to the situation.
The Celts inhabited many areas that were rich of raw materials like copper and tin (bronze is an alloy of those two) and iron ore, therefor they often used bronze and especially iron weapons, their shields were rectangular though they also had round shields with sharp edges that they could throw at their enemies.

During the zenith of the Roman Empire the Celts in Gaul and other parts of mainland Europe were eventually Romanized though the tribes in northern Britain and Ireland kept most of their original culture, during the Middle Ages all Celts were Christianized but many aspects of their original religion remained and were mixed with the new Christian religion, a good example are the Celtic legends about the quest for the magic cauldron that were changed into the quest for the holy grail (the cup from which Jesus Christ had drank during the last supper).
The legend of the grail is mainly about Josef of Arimathea who found the grail and was kept alive by it during his captivity in Palestine, he later took it with him to Britain where it disappeared because of the sinfullness of the people, king Arthur and his knights then tried to retrieve it but did not succeed, this story was especially popular during the Middle Ages.
The grail legends are connected to king Arthur who was not only a legendary figure but also a historical figure who lived in the period of the Anglo-Saxon invasions, according to the legends he has been killed by his nephew Mordred and is now in Avalon where he awaits his reļncarnation as the saviour of the Celtic people who will defeat the Anglo-Saxons.

Language: the Proto-Celtic language was of Indo-European origin and descended from the same root as Italic (Italo-Celtic), the Proto-Celtic language was later divided into Q-Celtic (which was spoken on the Iberian peninsula and the British isles) and P-Celtic (which was spoken on the mainland).
Nowadays Celtic languages are only being spoken in the more remote areas of Ireland and Great Britain where they are called "Gaelic", Irish Gaelic and Scottish Gaelic are very similar because the Scottish Highlanders are direct descendants of the Irish.
Examples of some Celtic languages:

(Universal Declaration of Human Rights article 1)
"All human beings are born free and equal in dignity and rights. They are endowed with reason and conscience and should act towards one another in a spirit of brotherhood."

Irish Gaelic:
Saoláitear na daoine uile saor agus comhionann ina ndínit agus ina gcearta. Tá bauidh an réasúin agus an choinsiasa acu agus dlíd iad féin d'iompar de mheon bhrthreachais i leith a chéile.

Scottish Gaelic:
Tha gach uile dhuine air a bhreth saor agus co-ionnan ann an urram 's ann an còirichean. Tha iad air am breth le reusan is le cogais agus mar sin bu chòir dhaibh a bhith beò nam measg fhein ann an spiorad bràthaireil.

Genir pawb yn rhydd ac yn gydradd â'i gilydd mewn urddas a hawliau. Fe'u cynysgaeddir â rheswm a chydwybod, a dylai pawb ymddwyn y naill at y llall mewn ysbryd cymodlon.

Dieub ha par en o dellezegezh hag o gwirioù eo ganet an holl dud. Poell ha skiant zo dezho ha dleout a reont bevañ an eil gant egile en ur spered a genvreudeuriezh.

Modern reconstruction of a Celtic sacrificial fire Religion: according to Celtic legends Ireland (Eire) has been named after Eriu, who was the daughter-in-law of the god Ogma, the island was originally inhabited by the Firbolg ("Sack-humans"), who were slaves from Thracia (the area between Greece and Turkey) where they had to carry heavy sacks of earth, they later rebelled against their captors and sailed to Ireland, unfortunately for them Ireland was conquered by monstrous sea-gods called Fomorii who enslaved them again, after some time the Firbolg and the Fomorii were both defeated by the Tuatha de Danann, who were a new family of gods.
"Tuatha de Danann" means "People of Dana" who was the mother of this gods, the Celtic word "Tuatha" is related to Germanic "Žeudo" and descends from the Proto-Indo-European word "Teuta".
The Tuatha de Danann were later defeated by the Milesians who invaded Ireland from Iberia (Spain and Portugal), after the conquest of Ireland the Tuatha de Danann moved underground where they changed into Elfs.
This legend may be based on facts since the early population of Ireland and western Britain (like Wales) was of Mediterranian origin and related to the Iberians.
The Scoti (the old Celtic Scots) were named after Scota, who was the wife of king Milesius (the king of the Milesians), the Gaelians/Gaelic were named after her son Goidel.
The Irish capital of Dublin was founded by the Vikings as "Dyflin", which was a Germanic adaptation of "Dubhlinn", the Celtic name for the surrounding area that was derived from a Celtic legend; there was once a female druid named Dubh who drowned the lover of her husband in a nearby lake and therefor the area was called "Dubhlinn" (Dubh's lake).
Speaking of druids; they did not only play a role in the Celtic religion but they were also concerned with advising, judging, teaching, diplomacy, sjamanism, and seething, there were both male and female druids and they were of such importance that they were allowed to speak first (even before the king) in gatherings and rituals, though it is known that not all Celtic tribes had druids.

The Celts also believed in an afterlife that can be compared to the Germanic Helheim, in Wales this afterlife was called Annwn ("other side"), and in Britain it was called Avalon.
The Celtic gods were believed to live in invisible worlds that could only be accessed in a soul-journey or via water or an entrance under a hill or well; on the evening before Samhain (October 31) the gates of the other worlds were opened which allowed the Elfs to enter this world (comparable to the Germanic Yule celebrations), Annwn (Avalon) was also an invisible world.

Description of the main Celtic gods:
  • Brigid (Irish), Brigantia (Britonic), Brigindo (Gallic), was the patron godess of the tribe of the Brigantes, she is connected to water, war, healing, prosperity, and fertility.
  • Belenos / Bel is the Celtic sungod, in Wales he was called Beli, in Ireland Bile, and in Gaul Belenus, the Celtic holiday Beltaine (May the 1st) was dedicated to him.
  • Camulos is a wargod and the patron of the tribe of the Remi, the center of his worship was Camulodunum ("Fortress of Camulos") which is nowadays Colchester, king Arthur's legendary castle Camelot has also been named after him.
  • Cernunnos is the god of hunting, forests, wild animals, and prosperity; he was depicted while sitting cross-legged with big antlers on his head, his name means "The Horned One" and this aspect has later been borrowed by the Christians who added horns to depictions of Satan, Cernunnos was mainly worshipped in Britain and Gaul.
  • Dana / Danu / Anu (Welsh: Don) is the Celtic mothergodess and also the mother of the Tuatha de Danann.
  • Epona is the godess of horses, in later periods she was adopted by the Romans as the patron godess of their cavalry.
  • Goibhniu (Irish) / Govannon (Welsh) is the god of smiths and one of the Tuatha de Danann.
  • Lugh (Irish) / Lleu (Welsh) / Lugos (Gallic) is a sungod and the god of arts and professions, he also invented the Fidchell game.
  • Ogma (Irish) / Ogmios (Gallic) is the god of eloquence and writing, he invented the Ogham alphabeth and gave it to his people just like the Germanic god Wodan gave his people the runes.
  • Macha is a wargodess who was forced by the king of Ulster (Ireland) to take part in a running match while she was heavily pregnant, as a punishment she caused the men of Ulster to have contractions for 5 days as if they were in labour; as you can imagine Macha is still very popular among feminists.
  • Manannan Mac Lir (Irish) / Manawydan (Welsh) is the god of the sea and the son of Lir (Irish) / Llyr (Welsh), he is also the patron god of the isle of Man.
  • Taranis is the thundergod ("taran" means "thunder" in Welsh and Breton), Taranis and the Irish god Dagda are believed to have been one and the same god, he was often depicted with a wheel and was one of the most important gods of the Celtic pantheon, he was not only a god of thunder but also a god of rain and the Celts brought human sacrifices to him which were burned alive.
  • Dagda (Irish) means "Good God" and he is the leader of the Tuatha de Danann, he had a club of which one end could destroy and the other end could grant life.
  • Teutases / Toutatis (meaning: "God of the People") is a wargod and tribal god, his name suggests that he may be a protector of the Celtic people.

    Ancient Celtic holidays:
  • February 1: Imbolc, this celebration was connected to the godess Brigid.
  • May 1: Beltaine, this holiday was dedicated to the god Belenos and on this day bonfires were lit (similar to the Germanic Ostara celebrations), humans or animals were also sacrificed by burning them alive in a reed puppet.
  • August 1: Lughnasadh, this holiday was named after the foster-mother of the god Lugh and is believed to have been some kind of Celtic Olympic Games.
  • October 31: Samhain, during Samhain the gates of the other world were opened which allowed the Elfs to enter our world.

    The Celts in Europe