The Roman Republic shall have seven sacred bodies: the Senate, the Consilium Principis, the Centuriata Assembly, the Curiata Assembly, the College of Pontiffs, the College of Tribunes, and the active Equestrian Order. Article III details the role and function of these bodies in the Republic and while other assemblies/colleges could exist, they had to be instituted by ordinary laws, and were not divinely ordained by Jupiter. They could be established or disestablished as necessary.
1) The Centuriata Assembly was an assembly composed of soldiers divided into centuries. Each century had one vote. In a departure from the present centuriata assembly, the process was wholly democratic and based upon proportional representation of numbers, rather than wealth. However across different soldier types, the level of proportional representation could vary: for example the Legions had a higher proportional representation based on their numbers than the Auxilia did. The precise level of proportional representation would be left to the Senate to decide. Every soldier had one vote in the century that represented them. Illiterate soldiers were allowed to vote as well. Any declaration of war accepted by the Senate had to be confirmed by the Assembly as well to be ratified. For voting purposes, a century is considered called when a majority of its votes goes in one way. As soon as over half the centuries support or reject a motion, the vote is finished. The traditional number of centuries was kept at 373, later the Senate would pass a law giving 180 centuries to the Legions, 132 to the Auxilia, 30 to the Praetorian Guard, 20 to the Navy, 6 to the Foederati, and 5 to the active members of the Equestrian Order. The commanding officer of cohorts, legions, numeri (foederati), fleets would preside over the vote in their unit.
Drusus' motive for making the Centuriata Assembly wholly democratic was that he believed wars were more likely to be won if the soldiers were wholly behind the war effort, and there was no better way of motivating them than to give them the right to decide when to go to war and when to accept peace. The Assembly would serve as a valuable link between the Senate and soldiers which would prevent the political and military classes from being wholly detached from one another. Lastly, citizen soldiers waived their right to vote or participate in any other electoral process other than the Centuriata Assembly when they joined the military. This was clearly an attempt by Drusus to get the military out of the political process altogether, as he was far-sighted enough to see the problem of soldiers disagreeing with political outcomes and taking matters in their own hand. By asserting beforehand that they were outside the process altogether via constitutional law, he was decreasing the chances that they might see themselves as the Republican's guardians or to have a great, vested interest in its politics. By trying to create a trend and culture of neutral detachment of the military from matters that did not concern them (those not concerning war and peace and the upholding of the constitution), Drusus would contribute greatly to elevating the professionalism of the Republic's soldiers.
2) The Curiata Assembly was a fusion of two prior Republican assemblies: the Tribal Assembly and the (ancient) Curiata Assembly. In the earliest days of the Republic, the Curiata Assembly functioned as the main democratic, plebeian assembly of the Republic, before reforms transferred that role to the Tribal Assembly. By reinstitution the ancient form of the Curiata Assembly and folding the rights and responsibilities of the Tribal Assembly into it, Drusus was attempting a call-back to the earliest days of the democratic Republic. However this was merely symbolism as he significant curtailed the powers of this body.
The new Curiata Assembly represented the citizens of Italy who were divided into blocs called curiae based on geographical location and population. Each curiae would represent roughly an equal number of citizens irregardless of wealth or status. Each curiae would get one vote before the Assembly, with there being 30 curiae in total, replacing the current 35 tribes of the Tribal Assembly. In the ancient days of the Republic, an Assembly would be called and all intended voters would congregate in one place for a prior convention where debates and speeches would be heard. Presiding magistrates had complete authority over procedure and could abuse it. Afterwards the voters would arrange themselves into their tribes/voting groups to vote.
To more effectively represent the citizens across the entirety of Italy, Drusus broke with the convention of a single, unified Assembly, and instead distributed voting across 30 geographical areas; but it would still be symbolically considered a single Assembly vote. While citizens could gather for conventions, they would be separated in different curiae to begin with due to geographical reality. Such split conventions would help foster a voting group identity distinct for each curiae. By the same token, geographically dispersed conventions would no longer be a part of the unified, assembly process; they were now considered the private, informal business of citizens outside the purview of the state. This removed the right of magistrates presiding over elections, to interfere with them.
However Drusus instituted a great number of safeguards against the dangers of democracy as he saw it. The voting period was limited to 3 days, and after the window for voting was over, and a plurality of its votes went one way, the vote for that curiae would be considered called. Each curiae was allowed only one poll station, so 30 voting stations represented the entirety of Italy. One had to either travel to a poll station personally to vote or hire a proxy, which advantaged wealthier voters who had such economic means and/or time to vote. (Essentially unless you happened to live near a voting station, you could not afford to vote unless you could afford a proxy) It also advantaged urbanites since voting stations were located in important population centres, as well as those who were better informed and had a more avid interest in politics since the voting period was very short and votes were frequent. All this contributed to a very low turnout even among those who were qualified to vote: male, literate citizens of Italy. Nevertheless compared to the old Assemblies, vastly more citizens were able to vote.
The main function of the Curiata Assembly was to elect "lower" executive magistrates: Quaestors, Aediles, and Tribunes. As Quaestors automatically became Senators upon the end of their term, the Curiata Assembly greatly influenced the long term composition of the Senate. But the Curiata Assembly had no judicial powers, legislative powers were limited to consultation and overview only, to gauge public opinion. It could not propose legislation and legislation could only be presented to the Assembly and thus the public AFTER it had been passed and approved by the Senate already and not before. Even though a vote in the Curiata Assembly on a piece of policy or legislation was a matter of consultation only with a non-binding result, nevertheless the Assembly was still widely seen as relevant due to the power of public opinion. It served as a minor check to the power of the Senate.
3) The Senate remained by far the most important body of the Republic, with almost all power concentrated into its hands. The Senate was responsible for electing all the "higher" executive magistrates: Consuls, Praetors, Proconsuls, Tetrarchs and the Princeps (more to be explained in Article IV), from amongst themselves. It was made up of all the most experienced politicians, statesmen, generals of the Republic who had to fulfil property qualifications to retain membership. This was a 250,000 denarii property requirement, though the Princeps could grant exceptions. Importantly, the Senate body was limited to 300 members only, with quorum being set at 200. Quorum was not required to pass acts and legislation, which could be done unilaterally by the Consuls. But it is required to open any debate and discussion over matters. Drusus disliked the notion of a large Senate since it made everything more difficult to get done, filled the body with useless members making it less prestigious and less competent and more corrupt, and also made it more raucous. If membership would exceed 300, the most senior members (by date of becoming a senator) were retired by the newcomers (Quaestors who finished their term of office). This was done to weed out those who were excessively old to carry out vigorous state duties and to stay informed of political affairs. This mandatory retirement would not include the Princeps, Consuls, Praetors, Propraetors, Proconsuls, Tetrarchs during their term of office. Since the Princeps and Tetrarchs (insofar as they were not dismissed by the Princeps) served for life, they were the only permanent members.
The Senate was the only effective legislative body, deciding and dictating both acts of domestic and foreign policy. Although in the aftermath of passing laws, treaties, it was not part of the mandatory process to consult the Curiata Assembly, it was the expected custom and tradition for major pieces of legislation. Senators by virtue of their imperium also have legal immunity and thus can only be tried by their peers. Only Senators may grant and hold imperium in the Republic, thus all proconsuls (governors of provinces holding the highest imperium in their province) were Senators also. This was also the case for Praetors/Propraetors, Consuls, the Tetrarchs, and the Princeps, who was princeps senatus (head of the Senate) in addition to being First Citizen. The minimum age requirement for Senators was set at 32 with no exceptions. The Senate as a collective body, was also the highest judicial authority (by virtue of imperium), though that power was rarely used. Ordinarily, the Senate functions in the judiciary process only by dismissing cases of corrupt higher judges from office as explained in Article VII, even if they did have the nominal right to overturn or decide any legal case by vote.
Lastly, Senators had to be actually present in a Senate meeting to cast a vote. (This held true for the Princeps as well to utilize his veto.) Those absent from Rome could not and thus successful Senators abroad would have diminishing political influence back in Rome, that held in check their ambitions abroad, and which would incentivize them to return.
4) The College of Pontiffs was in charge of religious affairs of the state, devoted to religious duties, the regulation and keeping of the Calendar, as well as the keeping of the Roman state cult of the 15 gods/goddesses of the Roman state religion. Underneath this body and in their charge, were various other religious bodies including the augurs (who read omens), the epulones (in charge of religious festivals), and the Vestal Virgins. The College was headed by the Pontifex Maximus elected by the other 15 Pontifices. Each Pontifice was put in charge of one of the 15 cults as priests/flamens. New appointments to the College would be given one of the 12 minor deities. The 3 major deities would be reserved for the promotion of a pontifice from a minor deity (after the election of a new pontifex maximus). A Pontifex Maximus was always chosen out of the 3 Pontifices in charge of the 3 Major Deities: Jupiter, Mars, and Janus/Quirinus. Membership of the college was stripped down to 16 from the 25 of Augustus, and membership was for life. Out of all the religious bodies the College of Pontiffs was the only one with a distinct political role.
Pontifices were appointed by the Princeps with the approval of the College of Tribunes (who could veto an appointment on grounds of bad character). They usually consisted of prominent, former Senators, newly retired. Though theoretically anyone could hold office, the Princeps would usually promise it to influential Senators in exchange for political favours. The 16 members had the role of electing censors, with the Pontifex Maximus holding the power of veto. Their secondary political role was the right to interpret the constitution to determine whether it had been violated in cases of uncertainty. Their interpretation had no formal legal standing, but had an important moral and religious standing as many Romans believed that only by following and complying with the divine constitution, would the gods look upon the Republic with favour.
5) The College of Tribunes consists of all former Tribunes who had served out their term. As there was a ten year required interval before Tribunes could run for office again, the College had a significant number of members, though it was unimportant politically. Their main role was to be often commissioned with specific tasks by the Senate, such as providing them with reports and briefings from fact-finding missions. They fulfilled this role as essential aides to Senate commissions in groups of ten known as decemvirs. Their only significant political role was the right to veto Pontifice appointments by the Princeps. Since Pontifices elected censors, and censors had the right to veto the censorial acts of the Princeps, the College had an important indirect role in maintaining the balance of power between the Princeps and the Senate. But more on that in Article IV.
6) The sixth body was the active Equestrian Order. While Augustus had instituted reforms allowing any man of sufficient property status to be called an equestrian, and the equestrian class filled important administrative roles, the Equestrian Order was reformed by Drusus into a purely military order. This was undoubtedly another concession to the Senate in exchange for various other reforms, but it had the effect of increasing the number of posts available to the Senatorial class, as well as for common citizens (with no property requirements) on merit. The active order would always consist of the current military commanders of the cohorts in the regular army (Legions, Auxilia, Praetorian Guard), who were appointed by a Senate military commission. All former cohort commanders were also considered part of the order as equestrians, but not as active members. It was stipulated in Article III that only active members of the Equestrian Order could command cohorts. The motivation for this on the part of Drusus was to better centralize command and authority of the army in the hands of the Republic's Senate, as opposed to potentially ambitious legati, proconsuls, tetrarchs, ect. It also provided for a more effective and trustworthy command structure for the command of Auxilia cohorts. More on this general topic will be discussed in Article VI. Apart from their military function, and their ceremonial role as part of a military order which was also established to foster a sense of military spirit and unity amongst the cohort commanders of the Republic to try and reduce the chances of civil war between armies, this body had no political role.
7) The final body was a new creation by Drusus and served another military role. The Consilium Principis was comprised of the two Consuls, the Princeps, and all ex-consuls who were not yet retired from the Senate who were available in Rome (having served out their term as proconsul in the provinces).
The Consilium functioned as a central military command (with the highest overall military authority in the Republic) or army general staff. Using the (highest) imperium of the Consuls, they decided where fleets, legions, troops should be prepositioned in the Republic. While all the former, ex-consuls were given no formal political role or power over proceedings, they had a voice in the council to give advice. The Princeps, by virtue of his censorial power, would have great influence over proceedings. The three most influential members of the body would undoubtedly be the Princeps and the two sitting consuls, who would make all the final decisions, thus the rule of three was formalized in their role as the current, ruling triumvirate of the Republic.
The effectiveness of such a centralized military command would prove to be limited because of the vast distances of communication involved. However it did serve to formally centralize military decision-making. Even though contingencies would still be effectively handled by proconsuls in their provinces, the Consilium was important for directing the movement of armies behind the scenes, to where they were strategically needed to reinforce existing troops and commands. Theirs was a strategic, grand-scale role.