Questions:
What's religion like in both regions? Any major ones? How important is religion in everyday life?
Overseasland: after the Dragon Cult lost it's state-religion status, became only a minor cult out of plethora of other cults and thought schools. Nothing became even borderline major in the last 150 years though, and the best you could get is a small temple in a major city.
Reich's populace is predominantly Periscanistic, aka follows the faith of the Healer Mother. There are priestly healers (who go around healing sick in exchange for a meal and/or shelter) and priestly preachers (who preach the good word and accept donations from the thankful no-longer-sick people), and mass prayers can be held both outside in open plazas or inside ornate cathedrals.
Periscanism is, of course, slowly taking ground in Overseasland.
Any major towns and cities that we should know about? What are they like? Anything they're renowned for?
They're just big population hubs. On the Reichland, you have Periscum, the seat of Periscanism, and Eurice, which surroundings are so blasted and polluted by the heavy mining/metalworks industry that it would make Sauron proud.
What's the average standard of living in Overseasland?
Is the rule of the Reich heavy handed or light? How are Reich officials generally perceived by the populous? Are Overseaslanders generally seen as second-class citizens compared to Reich natives?
The Reich is actively pushing for transforming Overseasland into a Reich-like place with skilled people living in cities and talent-less farmers tending to crops they sell to city dwellers for hefty coin. Overseaslanders might get funny looks in the Reich (as much as a guy from a village would get after taking a train to a city with skyscrapers), but on the Overseasland both are equal.
There is a nice division by generations when it comes to Reich rule; the youngsters will see it as benevolent because 'yay no corruption, yay no wars, yay stable economy, yay grandiose land projects'; their parents will clearly remember the invasion, and the elders will refer with sentiment to the times of early 4th century. Also, Reich did
very well on the PR side; while Troian nobles were squaring it off, Reich cleared the Wasteland, patched up lands shattered during/after Magic Wars, and provided much support during the natural disasters and civil war. You could say that some of the Troian populace welcomed the Reich invaders as their saviors, which is why they rolled over the mainland in a year and then took couple more to squash the other states in the neighbourhood.
A general description of the culture of both areas would be nice, along with how they've interacted or mixed.
To be honest:
The way I understand it, the basic culture clash between Overseasland and the Reich is that the former is (was) a feudal fantasy realm, with scheming nobles and warring dynasties, while the latter is an increasingly bureaucratic, rationalizing society. A bit like Westeros being colonized by 19th century Germany, except that they used geomancy instead of steam engines and Krupp artillery.
Is rather good explanation. Overseasland was more about spiritualism, noblemen and monster-hunting, while Reich was more about sciences, conquest and progress. Overseasland did try to do the same (the Council, the religion-free state, progressive ideas) but didn't really get anywhere due to natural disasters, civil wars and then the invasion by the Reich.