By lifestyle I mean the food they ate, weapons, clothes, architecture, religion, upbringing, how they elected leaders etc. Was it similar to the medieval period?
Also what was the population in these tribes? When they say tribes do they mean a couple thousand?
Hey there, I think that is a question I can answer. For this I will mainly use:
-Dougherty, Martin J.: "Celts. The history and legacy of one of the oldest cultures in Europe", 2015.
-Kuckenburg, Martin: Die Kelten in Mitteleuropa, Stuttgart 2004.
-Cunliffe, Barry: The Ancient Celts, Oxford 1997.
for the roman side I will mainly use bibliographie from my Roman seminars at University:
-Jehne, Martin: Die römische Republik. Von der Gründung bis Caesar, 3. durchgesehene Auflage München 2013.
-Rosenstein, Nathan/ Morstein-Marx, Robert (Hrsg.): A companion to the Roman Republic, Malden Mass 2006.
-Bleicken, Jochen: Geschichte der römischen Republik, 6. Auflage München 2004.
and since I've also written a bit about the iberians I will use:
-Keay, S.J.: Roman Spain, California 1988.
-Luik, Martin: Der schwierige Weg zur Weltmacht. Roms Eroberung der iberischen Halbinsel 128 -19 v. Chr., Mainz am Rhein 2005.
Apologies in advance if my English grammar is not perfect, I'm a native German speaker.
First of all, most written sources about the Celts are from Romans who weren't always very reliable when writing about their "barbarian" neighbours and who often liked to exaggerate a bit. To give you a clear answer I will divide my text into categories. I will mainly focus on the Celts in Gaul (and Britain) and might say a few words here and there about the Iberians and Germanic but I don't know as much about their culture as I know about the Celts (I mainly know a few things about their warfare and settlements).
But I have to make one thing clear first: Celts existed for a long time in a big territory so it should be obvious that not everything I say is applicable to every tribe.
-2100-800 BC Hallstadt bronze age in todays southern Germany -800-470 BC Hallstatt iron age in Central France, Western and Central Iberia, Southern Germany, Switzerland and Southern Britain, -La-Tène from 450-50 BC in the same areas, plus parts of Scotland, Ireland, ancient Scythia and Galatia/part of Asia minor.
-"Goidelic" (Irish, Manx and Scottish Gaelic)
-"Brythonic" (Breton, Cornish, Welsh) Celtic.
Food: The Celts relied on agricultural farming, livestock farming, fishing and hunting and their standard of living, burying, art and their ability to establish trad etc. shows us that hunger and food supply probably weren't really big problems. In the Iron Age they were able to craft iron ploughshares pulled by oxen which increased their yields of crops. But like the Romans, they relied on salt, not just to preserve food but also for profitable trade (salt was very valuable). What they ate specifically: Vegetables (like leeks, onions, carrots),nuts, berries, bread and porridge from grain, garlic, spinach, meat (of course: deer, boar, bear, fish or domesticated animals like chicken, goats, sheep, pigs), eggs, mushrooms and honey. Romans, depending what they had conquered, pretty much ate the same, with addition to dates, pears, plums, pomegranates, olives, apples, pumpkins, grapes and cheese.
weapons: The celts were skilled in craftsmanship of not only jewellery and decorations, but also of weapons and armour. You had chariots as both a means of transportation for warriors into battle (like in Caesars invasion of Britain) and as a status symbol for warriors and chieftains. They also had very common weapons like spears in very different shapes, a shorter javelin (gaesum) which were often used by celtic youths who weren't quite ready to fight in a melee fight, slings, bow and arrow (though rarely and probably mostly for hunting), knives, swords (with rounded tip, so probably used for cutting rather than stabbing. However the spanish shortsword, used by the celtiberians, was a stabbing and slashing weapons and was adopted by the Romans). The celts used big painted oval and rectangular shields with a buckle in the middle.Those who had armor used either leather armor reinforced with metal, or mail shirts. Helmets had cheek and neck guards and weatlhier warriors decorated the top of their helmets with horns, feathers, animals (like imitation of a raven or boars of metal etc.) In addition to that the Celtiberians used breastplates and curved swords (falcata). You might see that Roman soldiers, especially during the Repbulican era, before using the lorica segmentata, looked a lot like Celtic warriors. The helmets (both the Montefortino and the GALLIC helmets) had originated from Celtic helmets, their oval scutum looked a lot like celtic shields, their shortswords were Iberian and their small breastplates and mail also came from the celts.
Clothing and body care. Clean clothing and good hygiene were important to the Celts. Though they had dirty and roughspun clothing for working in the fields, their everyday clothing said a lot about their prestige and personal care. Their high quality fabrics were quite popular among other cultures they traded with. Celtic clothing was very colourful (mainly red, yellow, blue, green and different shades of grey) with diamond, checkers and striped patterns. Of course they used materials and fabric which they got their hands on depending where they lived (leather, wool, hemp and animal hair). Shoes were made of leather but also had wooden or cleated soles. Very important though were their pants, or Bracae, (something that the Romans often remarked and what they found to be quite barbaric) which were also quite colourful. Women wore dresses but probably also wore pants for riding. Both however wore coats and cloaks made of wool or leather and fixed with brooches. I'm sure you know generally how the Romans dressed: They had tunics (m) and stolas (f) and cloaks as a basic wear made of homespun wool and linen while Togas (m) and Pallas (f) were for the wealthy (cotton and silk). They wore cleated sandals or went around barefoot. Although pants were later adopted (at first by the soldiers stationed in northern regions), at first they were seen as barbaric.
architecture My last point for now. Religion, politics, law and society are such huuuuge topics that I'm going to have to make an additional post. The celts lived in huts; these could be round or rectangular and the basic structure was marked with wooden pillars which carried the roof. The walls between the pillars was constructed with wattle and daub and filled with plaster. The roff was made of straw that nearly went down to the ground to give shelter from rain and wind. For the wattle they used willow and hazelnut and put on the daub (mixture of straw, clay and straw). At the end huts were probably chalked white to either look better and cleaner, and to reflect light and heat in the summer. Apparently those huts were surprisingly breezy and not as dark as you would think and also quite comfortable. Villages were protected by smaller earthworks with palisades on top and trenches in front of it. For more protection they used hillforts (Numantia in Celtiberia was a good example how secure they could be as the Romans tried to conquer it for like 50 years until Scipio Aemilianus finally succeeded to starve the Numantians out with a giant army). They were also surrounded by trenches and earthworks but were constructed on strategically advantageous positions, (like hills, mountains, forrests or surrounded by rivers) and the walls could be reinforced by stones (at areas were the walls were not well protected by natural obstacles and barriers). These were obviously most effective against other Celtic tribes, raiders etc. and not that much against a proper Roman sieging force, however, they were surprisingly effective against the Romans as well (like Numantia or Gergovia). Those forts however were like later castles. They couldn't hold the entire population of a tribe, they were rather a place where an army could fortify itself and wait for an enemy army to attack or go away after plundering the countryside.
Like I said, that will be all I write for now, but I'll probably come back to write about the other points you mentioned. If you have any questions to the stuff I just wrote, then please ask :) I hope it was already helpful