Oh, quick irresponsible workday note on the Royal/Senatorial thing.
So both castes/social strata make up Taldor's upper crust, aka the Bearded (called such because if you are a Taldan, only men of the bearded get to have beards, and are expected to have some well taken care of growth there, be they some full out dwarf thing or a carefully groomed chin strap. Foreigners and demihumans are exempt. Basically it's a sign that you have the time and luxury to, well, take care of a beard of some shape).
The main difference is that the royals are, well, royal. The Grand Prince, (the head of Taldor's title) comes from the royal nobles, and particularly when the current one has no male heirs, one gets picked out from the other royal families some way or another. While a decent chunk of them can trace descent to the founding of Taldor or close enough, an even bigger chunk of royals got their status from being given it by some past Grand Prince or another for some accomplishment or another, though this is done only /just/ frequently enough to leave it a semi plausible dream in the eyes of the underclass. The royal families are at this point many, often rich as fuck, and in many cases indolent/decadent/corrupt. On the other hand, they also provide a lot of the Taldan military as far as house forces that can be rallied in a pinch, and a good chunk of the officer corps besides (not always the quality officer corps, but hey).
The Senatorial class are called such because, well, they sit in the Taldan Senate, in the magnificent old Senate building they often go in the back of rather than take the many tiring steps that are supposed to be a reminder of duty. The Senate has more or less the power the Senate did in the Roman and Byzantine Empires, aka as far as determining national policy, sweet fuck all usually. /But/, the Senate does control and manage the (corrupt, bloated and inefficient) imperial bureaucracy, which is where the caste gets most of its power from. So the Senate meets and debates about that kind of thing, and on whatever other issue they feel like (one of the big acrimonious debates in the Senate is which national enemy of Taldor should be focused on, Galt or Qadira). There are certainly ancient senatorial families that go back to the founding, buut, the Grand Prince will more frequently elevate a commoner to senatorial rank for whatever reason, making it even more of a "work hard, keep your head down, be a good little Taldan, and someday even you!" and so forth. They're rich, though not quite so rich as the royals, and powerful, but they more wield their power in blocs. They're the lesser nobility, more or less.
Exceptions exist certainly. There are certainly senators richer than royals, and royals who manage to get hooks into the bureaucracy, but on the whole, that's the shape of it. There can and are fierce rivalries between members of the groupings, but considering the more sweeping societal prestige of the royals, they can sometimes be one sided (but then again, sometimes not).
Both groups will totally defend their priveleges like whoa, regardless.