it might also be because of the clothes he was wearing, which to me look like it could be celtic.

The history of the pre-Roman Celts can be written-- not just from the stories of those who encountered them, namely the Roman Republic (and earlier the Hellenistic Greek states), but from the archaeology (in the absence of any extended narrative accounts by the Celts themselves). The best man to read about Celtic Gaul right now is Christian Goudineau (e.g. his book on "Cesar et la Gaule", but he's translated e.g. in Cambridge Ancient History volumes). Older is Paul-Marie Duval (v. criticised now); even older, e.g. the French archaeologist Déchelette, and Camille Jullian's turn of last century History of Gaul.

Anyway, some pointers:

Hallstatt princely cultures (C6th), large settlements
Early La Tene, hill fort cultures, around principalities (say ca. 200 BC)
Late La Tene, trasnformation (end of principalities, social tensions, linked with social transformations notably caused by contacts with Rome, slave and wine trade, pressure from Germanic tribes) (say 100 BC- 50 BC).

But it looks a bit different according to e.g. local scholarly tradition, but also different developmental rhythms (e.g. Britain kept the war-chariot, with the concoitant social organization, much later than the mainland).

Best museum for all this: Saint Germain en Laye, near Paris.
Best site for all this: Bibracte, in Burgundy.

here endeth the lesson

http://www3.hants.gov.uk/countryside/danebury/danebury-history/danebury-ironage....

Life at Danebury in the Iron Age

Life was short and harsh in the Iron Age. Danebury was predominantly a farming community, the people kept sheep and cattle, wove woollen cloth and made leather goods. As Danebury had few natural it relied on trade with other areas to obtain iron, tin, copper, salt, shale and stone. It is likely that woollen products and grain were traded in exchange for these.

A community of 300 to 400 people lived here for more than 400 years. During that time one of the people’s main tasks may have been to protect livestock and grain from attack by raiding parties. Men, women and children may all have had to fight off invaders by hurling sling stones. Warriors fought with swords and sometimes used horse drawn chariots.

At the highest point of the hill there were shrines and temples. Religion was important to the people who lived at Danebury.

Their pagan belief was that the gods lived in rivers, trees or other natural features. They made offerings to the gods and sometimes sacrifices. Some of the burials uncovered at Danebury are thought to have been sacrificial. These rituals were carried out by the priests, known as druids, who were respected in the community and acted as a link between the people and their gods. They were also law makers, teachers, storytellers and medicine men.

Beneath the modern fields lie the remains of smaller ancient or Celtic fields. Aerial archaeology has allowed us to map these systems, which appear as ‘crop marks’ or different colours in the soil. It shows that the farmed prehistoric landscape was just as busy as that of today.

http://www.academia.edu/7998513/Reassessing_slingstones

On the basis of this review, I contend that many recovered slingstones from these regions are nothing of the sort. Neither the case for, nor the case against them, however, is without difficulty. For example, while it is easy to demonstrate that a group of beach pebbles is or is not well-sorted, and thus perhaps curated, sorting per se is not in fact a prerequisite for effective slinging (Brown Vega & Craig, 2009); rather, it is necessary for us to show that assemblages incorporate stones that fall inside or outside a range attested as effective for slinging in the ethno-historical record — between about 28 grams, the size of the smallest Roman glandes (lead sling-shot) (e.g. Greep, 1987: 183), and just under a kilogram (2 lbs), attested for slingstones in Tahiti by the first western navigator to reach that island (Captain Wallis in Hawkesworth, 1773: 445). Secondly, owing to our incomplete knowledge of the Tertiary geology of these regions (Maiden Castle is unhelpfully recorded on the geological maps of the region as ‘made ground’), we cannot always demonstrate whether stones have been imported onto a site or not. Thirdly, beach pebbles are intrinsically undateable and we have to be very cautious in interpreting their final archaeological relationships. Finally, the process of excavation has destroyed the features from which these stones were recovered and it is often necessary to reconstruct their associations — feature, sedimentological, artefactual — from what is a very incomplete record. For some assemblages from these regions the available evidence is so equivocal that it is impossible categorically to rule in or rule out an artefactual interpretation for them. Nonetheless, the evidence against their identification as slingstones is compelling, certainly enough to raise a 'reasonable doubt', and — in my view — enough validly to challenge the established orthodoxy elaborated above.