The government of Keltoia was largely a dictatorship, lead by the Emperor in the capital of Nouio. However, after the phenomenon of Ambactoismos, the government became one of the more decentralized Empires in the world, with many parties having some sort of influence or stake in the structure. Instead of ruling the nation directly, the Emperor would need the approval of his Chieftains and their Chiefdoms all of whom held a substantial amount of influence over the politics of the country. In fact, it is fair to reduce much of Keltoic politics to a turf war between the central Emperor and the less centralized but still powerful Chieftains. In order to do anything, the Emperor needed the support and approval of his Chieftains, while the Chieftains needed the Emperor to support them, lead their armies, and keep the nation cohesive enough to avoid to much infighting.
Making the entire system yet more complicated were the cities and the Senate. The four major cities - Nouio, Dubron, Parisi, and Locudula - often tried to manipulate the Empire to work for them. Because the cities were most populous centers in the Empire, they were able to wield their influence very effectively, especially in terms of trade and other economics. In addition, the cities could sometime relay on their traditional power - after all, when the empire was founded, they had really been all the empire had. Meanwhile, the Senate ostensibly served as an institution of the people, but effectively served as a reflection of the will of the upper class. With these forces at the table, it was often hard for the Keltoic government to operate, and even when it did, some level of infighting was inevitable.
The Emperor, despite much of his power being ceded to the Chieftains, was still the most powerful figure in Keltoic politics. While he only technically had control over Keltoia, he could easily assert influence over any other chiefdom, at least to some extent. For example, he, along with the Senate, was in charge of setting basic taxes for the entire nation. Chieftains could of course set taxes on top of these, but they had to be approved by the Emperor. In addition, the Emperor had a lot of influence of trade. Anyone who wised to be a merchant within the Keltoic empire needed to get permission from him. In practice, this permission was almost always indirect, but it still gave the Emperor a lot of control over what nations could trade within his borders. Between the control of taxes and a thumb on the scales with trade, the Emperor had a lot of control over the Empire's economy.
Besides the Economy, the Emperor had a huge amount of influence in one other area: the Military. While all of the Chiefdoms had some form of standing army, the only one with the authority to call for any war outside the nation's borders was the Emperor. In addition, when troops were needed to fight a war, it was rare for the troops to be on hand, even when all the Chiefdoms polled their resources. Thus, it often fell onto the Emperor to raise an army, with some help from his Chieftains. Outside of raising the army in times of need, the Emperor was also responsible for funding it. In general, this simply meant appropriating taxes to fund the military, though on some occasions Emperors were recorded borrowing from other nations to fund the military. All in all, the Emperor of Keltoia had two main sources of wealth and power - their influence in the Keltoic economy, and their control of the Keltoic military.
The Chiefdoms and Chieftains
In Keltoia, the Cheifdoms equate to provinces, each one being under control of a Chieftain. The Chieftains generally had the same set of powers as the Emperor, just on a smaller scale. This essentially meant that Chieftains had control over taxes and trade in their chiefdom, as well as some control over their military. While the Emperor could set taxes on the entire Empire, the Cheiftains were limited to taxing their own provinces on top of that. However, the obvious limiting factor of this power was that the Emperor's taxes were always on the province, so by adding more, Chieftain's risked some level of dissidence. Also, Chieftains could allow merchants to trade in their chiefdom, even if they did not have permission in the rest of the empire. In practice, this rarely happened, though it was more common in South Keltoia, and Mediterranean.
Chieftains also had a fair amount of control over their Chiefdom's military. While only the Emperor could call wars outside of the empire's borders, it was left to the Chieftains to defend their territory. This meant that while the Emperor provided funding for the military, it was the chieftains who were responsible for their day to day operations. For example, Chieftains generally chose were forts and strongholds were built, and also had troops protect major trading posts. On the few occasions that Keltoia was invaded by Hostile forces, it also fell to the chieftains to manege the defense, at least until the Emperor could organize something more cohesive. However, it is worth noting that the Emperor usually held the upper hand in matters of the military.
The Cities and the Senate
While the Chieftains and the Emperor were technically in control of the nation and didn't have to take into account the interests of either the Senate or the four major cities, in practicality, their input had to be weighed for almost every decision. The Senate, while its power could easily be curtailed by the Emperor, was still influential. Not only did the Senate have the power of hundreds of wealthy families behind it, but its elected members helped serve to legitimize the government in the eyes of the people. Because of this, the Emperor would at least have to consider laws put forth by the Senate. taxes were also influenced by the senate, and they often submitted recommended tax levels. However, the Emperor did hold a fair amount of influence over the senate because of his control over the military, and their occasional crackdown on Senate dissidence.
The four major cities probably had way more influence than they had any right to in the government of Keltoia. Not only did they control a huge amount of the Empire's trade, but they also had a fair amount of their population. Because of this the Emperor, and in Locudula's case, the Chieftain of Locudula, had to go out of their way to meet the needs of the cities. Most of the time, the cities didn't try to influence the course of the empire, but rather maneuvered to make sure sufficient resources and investment were going towards them. It was only when the cities were not happy that they became a thorn in the sides of the Senate, Emperor, or Chieftains, and this was rare. As a result, the cities were largely minor players, but they had a lot of weight to throw around if they wanted to.