At the end of the world, there is a great people. Far removed from the wars of the Mediterranean sea, the steppes, and far off deserts, they wage their own war. Your people are among the most fearsome in all of the known world, and you know this. They came to these islands and crushed, displaced, or flatly annihilated the old people; broke them utterly. They are poets, and traders, and miners, but most of all, they are warriors. The people of the islands cry to you. They want a leader. They want some one to direct them, to lead them to great things. They want you, and who are you to deny them? This land is your right, it is the right of anyone strong enough to take it. Would you let the petty lesser tribes put you down before your chance for greatness?
The British tribes have always fought as skirmishers and ambushers, with only the rare field engagement. While this is fine for petty tribal conflicts, a potential emperor of the Britons cannot focus on this forever. He must be decisive and swift, and build a real army, not an amalgam of petty warbands. But you are the ruler of the Casse, and they have long been the most militaristic fore-thinking of the Britons. They are descended from Gauls, and employers of Gallic tactics; with planning, Britain will easily be attained, barring any unforeseen circumstances. Britain is rich; rich with tin, and silver, and copper, and livestock of all kind. A large, well funded army is quite possible, with all of the island under your yoke. The southern tribes provide the strongest, heaviest soldiers. They are the best trained, the best equipped, and the best disciplined tribes. The midlanders are more wild, lightly armored at best, but available in great number, and are relatively inexpensive, making them a good asset for overcoming an enemy by number. In the far north lay the remaining peoples. Caledonians are without remote concept of civilization. Abhorrent even to the other Britons, they are maniacs in many ways, and completely disorganized. However, they are brave. If nothing else, Caledonians are an effective shock force, and fairly disturbing to an enemy. You must rely on infantry and foot soldiers. Your cavalry is scarce and weak, and chariots are expensive. You must organize units of champions, brave warriors, who will encourage your other men to continue to fight. Britons are brave, but sometimes practicality of a situation overcomes them. Seeing great warriors of their people continue to fight will spur them on though, but champions expect compensation.
To become high king and forge a real empire, uniting Britain will be the foremost part of your early reign. The Britons are pleased to serve a powerful king, so you must show military strength. However, they also expect acumen in business matters; Britain has always been made rich by trade with distant foreign powers, and they have come to enjoy their lifestyle. You are not ready for wars with any major powers. Not yet, anyway. It may be humbling, but you are, after all, only a very small kingdom. Conquest of all of the British Isles will provide you with an easily defended enclave, which will produce great amounts of money, with proper trade agreements, and a large population from which to pool a great army of conquest. Invading mainland Europe will not be easy, but if one takes advantage of the constant wars there; attacking weakened enemies in Gaul or Germania, you can secure an easily reinforced stronghold on the mainland, and expand from there. However, this assumes everything goes to plan, and no one turns a covetous eye on your wealth. Germans are strong, brave, and numerous, and Gauls have great soldiers and relentless leaders. Further off are the ambitious Romans, Carthaginians, and the hearty Hellenic peoples, who already know the value of the island from centuries of trade. The task is daunting, to say the least, but you are not a leader of common men. You are king of the most fierce warriors, wisest clerics, and greatest champions on the whole of the planet. You are the rightful king of all of Britain, and your people deserve an empire. Their lives have been endless strife and toil at war. They deserve an empire to reflect the greatness of their achievements. As your glorious ancestors drove the wretched men who once inhabited the island from it, as they had destroyed them, so must you destroy your enemies. You must be vicious, you must be relentless, and utterly break every enemy to your will. Your people deserve peace, and peace will only be had when your enemies submit to your rule. Again, king of the Casse, I ask you, who are you to deny them?
The Casse came to Britain in an unknown year, but likely in the third or fourth wave of Celtic migrants from Gaul. They established themselves as a military and trade power swiftly. They traded with Gaul, the Greeks, and likely many others. They were very similar to Gauls in many respects, like most southern Britons, and had some influences from the Belgae as well. Archaeological evidence, and transcribed oral histories, copied in later periods by Christian monks, point to stories of a failed attempt at unifying the whole island under their rule. A plague racked the Casse and their subject tribes, among them, the Atrebates, Icenes, Trinovantes, and others. The plague was seen as a bad omen, as the king and several sub-kings had died from it, so the attempt fell apart. Caesar recongized them as the Catuvellanians, and called their king Catuvellanius, likely taken from an actual Gallic title; 'Catuvallanorix', or 'King of the Islanders'. At any given time, their power waxed and waned in varying levels of severity, but they remained some of the best organized, strongest, and most wealthy of all Britons. Their control of tin trade, large silver reserves, and substantial wheat harvests ensured them a place as the most powerful in Britain.
Culturally, the Casse were very similar to the Gauls and Belgae, with distinct Briton influences. They had advanced coinage, large mines (significantly larger than Gauls or Belgae, for that matter), quality metallurgy, and an involved political and legal system. The average person's life was spent pursuing a trade, going to a school, becoming a soldier, or going into politics, not unlike a later society. Day to day life for Britons would be a bit more stressful than it was for their Gallic counterparts; the threat of enemy tribes raiding or invading was constant. However, that doesn't mean every moment was purely war. A favored pastime was sports, including a game we now call 'hurling'; so popular was this game in all of the British Isles, that it's mentioned in numerous legends. The Irish hero Sentata, better known as Cu Chullain, was noted for his great skill in hurling. They also told lengthy stories, sang, played music, and created poetry, about such heroes, wars, and gods. They engaged in large public feasts, keeping the local chiefs and chieftans as somewhat accessible characters, to whom the people felt an honest relation. This practice encouraged a tribe to feel exemplify their relations; the extensive Celtic clan model encouraged loyalty, so long as the people felt they were truly related. They were also good farmers, and had a strain of wheat grain that was exceptionally healthy, pointing to an advanced understanding of farming methods. They also ate a great deal of meat, especially compared to later civilizations in the region, which likely accounted for their great size; they were even taller than Gauls. Swords were not as common among the Britons; iron was expensive, and often imported from Gaul or the Goidils in modern Ireland. Iron was favored, instead, for spear and javelin heads. Swords in the south were often imports from Gaul, and swords in the midlands were made to be shorter than other Celtic swords, to preserve iron. Everyone in a tribe was considered family, even slaves, who eventually worked their way into the upper tribal community. Their economic model was a type of anarcho-capitalistic lifestyle, with religion encouraging charity, but never enforcing it. Taxes were taken mostly to provide their leader with a home, improve settlement defenses, and pay their warriors and champions. Soldiers were paid based upon experience, and it was not uncommon for a particular warrior to recieve a large gift for performing a heroic action, such as a pile of silver, or a new weapon or a shirt of mail. In war, champions were a rallying point. They would be banded together as an elite group, whom which the other soldiers would unite with, and follow them into battle; this was an important aspect of British-Celtic war. While the Gauls were similar in some respects, the Britons relied heavily upon champions to inspire them in battle.
As in any Celtic kingdom, their leader is elected, not hereditary. The tribes elect a chief, chiefs in an area elect a chieftan, chieftans elect kings over a larger area, and the kings elect the high king. That is, in a stable kingdom. The Gauls and Goidils successfully managed larger kingdoms in this format for many years, but the Belgae and Britons struggled to attain a remote semblance of order over anything but the smallest of areas. Part of it would have to be enforced through pure military might, to ensure every tribe and region plays along, even when their king is not elected as the high king. In terms of law, the kings had little power; law lay in the hands of judges, elected as well by the people. The higher one's station in society, the harsher the fines and punishment under the law levied against them. Law was central to Celtic religion, and elected officials were meant to be exemplars of the law. A king was a military and business leader, and expected to be intelligent, versed in several languages, and capable of fighting in combat, and intent on leading his men personally when the chance was logically available. The Casse had a king, who ruled over the Casse and their surrounding subservient tribes, such as the Belgic Atrebates, directly. His position was mainly as the organizer of the military, and had a great deal of control in this respect. He also had control of diplomacy, and was the final word in making or breaking alliances. His electors were a kind of 'senate', composed of all the chiefs and chieftans under his rule; after his election, they would also act as advisors. Chieftans and lesser kings acted as generals and lieutenants, and chiefs were captains of local militias. As such, all of them, from the lowest chieftan, to the high king, were elected based on a multitude of issues, but the foremost was their ability to command and lead soldiers. The elected judges formed a seperate 'senate', through which they passed and modified law. This was a slow process; this model was still in use in post-Christian Celtic countries, such as the Irish and Welsh kingdoms, and it's notable that almost no laws changed over hundreds of years. A change to a law would be proposed, and the judge would take this proposal to his tribe. The tribe would vote for or against it, by majority rule, and the judge would return to the conclave, and give the tribe's results. A majority of votes on part of the judges' tribes was required to pass or veto the proposal.
The religion of the Casse, and other Britons, involved dozens of minor local gods, demigods, hero worship, and major deities that would be worshipped over huge regions. Most of their deities tend to be war, health, or legal gods, and their heroes tend to vary between warriors or great poets and storytellers. Among their most important deities are Camulos, a Belgic god of destruction, and Andraste, a British goddess of war. Both require sacrifices, and encourage a large amount of xenophobia toward outsiders; though Britons were remarked as cordial and even warm to outsiders; as well as absolute obedience to the law. Their religion also includes certain ritual aspects, such as painting the body, believing it provided some amount of special, magic protections. They also collect the heads of dead enemies, not just as trophies or signs of bravery, but because the belief that the soul resided in the head. To control a man's head, they believed, meant that his soul had to be your slave, in both this life and the next. The 'druidae', druids, actually didn't come to Britain until around 70 BC, though they'd been in Gaul much longer. However, an essentially identical class, which we casually call druids, was present before hand, and their influence was quite strong. If a higher official was arrested, such as the king, this conclave would also try him, collectively, rather than having a single judge oversee him, since he represents all of the tribes. Likewise, a chieftan would be tried by those judges of the tribes he represents, if he was accused of breaking the law. These systems meant to give each tribe a voice in the events of their kingdom, and the matters of running it.
The Britons were conquered, allied, or otherwise subverted to the Roman powers. During the Roman conquests, Britons fought both as defenders, and alongside the invaders. The Romans withdrew due to constant pressure from both Picti from Caledonia and Goidils (called Scotti) from Ireland. The destruction of silver mines, tin complexes, and encroaching Goidilic slaver colonies and piracy operations made Britain far too unprofitable, especially considering the problems in the other parts of the empire. The Britons continued to exist for a very long time in varied sub-cultures, such as the Cymriae and Cerniuae (Welsh and Corns), the Strathclyders, the Regyddites (who were also partially Gaelic), and the Cumbrians (also had a large Gaelic population). Even today, the Welsh remain a distinct people and are culturally descended from the Britons, though diluted heavily by the Anglo-Saxon and Norman influences from England, and Gaelic influence from Irish colonies established during the dark ages. The Britons never experienced the total Romanization that Gaul did, and their culture was quite resilient and long-standing because of it. 
The Casse rely on skirmishers in early armies, but progressively more shock infantry. They support their armies with chariots, light horsemen, and most importantly, 'champions' of various types. Most of their units have lower than average morale, but many higher upkeep units raise allied morale, to encourage the Britons to continue to fight.
This unit list shows Casse troops and regional Briton units. Heavier infantry and other units can be raised from regions outside Britain, like Goidlic infantry from Ireland and Belgae cavalry from northern Gaul.
The Casse campaign may have humble, quiet beginnings as far as glorious hosts of warriors and battles between major factions is concerned, but I tell you now, it is very much worth it.
You start the game headed into debt, with a single city and an army that is much too large to be supported by it. To top it off, you have a fleet of ships with a massive upkeep floating off the southern shore.
Your first step, during your first turn, should be to get your diplomat across the channel and onto mainland Gaul. Once this is achieved, disband your navy. You will not need it for another decade or two. This will save you around 1500 minai per turn.
Next, you have the option of consolidating your army and marching north on Ratae, possibly taking it, and then disbanding unnecessary units to build up your economy. You could do that – but it would be a bad idea. Here’s why – unlike any other faction in the game, Casse does not start off an enemy of the Eleutheroi. In fact, you are even trading with them from the get-go! Don’t mess this up by attacking them before you’ve stabilized and amped up your economy – especially since they have the habit of attacking right back! Disband all unnecessary units along with your ships on your first turn, and concentrate on improving your capitol city. There will be plenty of war to come.
Now, don’t forget about your diplomat! Send him throughout Europe, getting trade rights from any factions you come across. You will need these treaties, so don’t forget to do this.
When you are finally ready to make war on your neighbors, I suggest taking Ratae and Caern-Brigante first, as soon after one another as possible. If you have trade rights with the Sweboz and the Aedui, etc., you will start raking in gold relatively quickly. Ictis should be next on your hit list, as it is the first city you come into contact with that gives you access to ship building, which will be crucial - so build Ictis up now rather than later!
Ynys-Mon is the final stop on your initial blitz war before your steady march into Caledryn and the Emerald Isle. This will end up being one of your biggest money-making cities, especially when you have conquered Emain-Macha. Once you have conquered the entirety of the British Isles, you will have a secure, profitable base of operations from which to expand your glorious Celtic Empire. You can strike out at the mainland anywhere you want. I would personally suggest conquering the northern coast of Gaul first, particularly Bratosporios. These regions have the best possible government options, familiar culture, and good units for your armies. Additionally, these regions allow for the easiest and quickest shipments of reinforcements from the Isles.
But really, you should have the money and the naval power to invade anywhere you like, so long as you can commit to expanding on the mainland from there. Just pay attention to diplomacy before you make you first enemy – you want to cover all your bases and avoid pissing off the wrong factions by attacking their friends. After all, you want them to trade with you up until the moment you are attacking them directly!
In case you haven’t guessed, the Casse are quite big on sea trade. Build accordingly. They love extra trade routes granted by improved ports, as well as markets and roads. These things should be at or near the top of your build order in each city. Play the Casse as the warrior-merchant kings they are meant to be, and you will have no trouble fielding multiple armies against your enemies and falling back on mercenary support at a moment’s notice! On top of that, you can lubricate diplomatic transactions with all the gold you want. It is not uncommon to have 200,000+ minai to play with each turn.
Once tactic I’ve has success with – and also fun with – is keeping to the coastlines, setting up strong coastal fortresses and not conquering too far inland. Instead, beat the crap out of the locals until they agree to become your protectorate, then use their lands as buffers between you and greater enemies, like the Romani and Carthage, while you push east or west as desired, soaking up gold from all the coastal cities in western Europe before sweeping over your remaining enemies with an endless swarm of warriors. It can be great fun when played correctly, so long as you don’t mind ultimately betraying your protectorates in order to achieve victory.
The Casse are all about hard and fast offense. Lacking regular heavy infantry and tactical flexibility, they must rely on powerful charge of their units and the good number of morale-boosting units as well as fear inducing units to quickly terrify the enemy and overwhelm them.