I vote that the empire controls, at most, the planet. Namely because our currently agreed scope is upon the planet itself and with the solar system being part of the backdrop. Not that we couldn't expand into that, but I'd like to start with the planet itself. Perhaps our empire is working on finishing conquering the planet, if we go with the suggested conquest idea, with our eyes already set on the horizon of the solar system and beyond. Naturally, it won't go as smoothly as all that.
Culture-wise, I'd fall in line with the idea of noble houses that the king/queen/emperor/empress needs to continuously court in various manners so that they can keep armies on the frontlines and avoid rebellion. To further implement this, perhaps install a caste system that our society has to uphold. Meaning that, if a soldier caste is implemented, moves like simple and sheer conscription couldn't be done except in emergency. Which leads to the leader of our empire/kingdom unable to raise a mass number of forces easily. Thus, they must court the various noble houses for their caste members. Naturally, breaking from your caste, in a manner that still has you as a part of society, is very difficult.
Furthermore, this builds into
@Draco Shadowdragon ideas on morality. Education, good morals, and hard work are now framed in being the best member of your caste you can be. To put a spin as to why drugs are illegal, let it have been discovered, or manufactured, a drug that radically increases a workers production ability. However, it gradually killed the user. As one of the greatest triumphs of the worker caste, they were able to ban the drug, and horrible drugs in general, throughout the empire. No more than a century ago. Of course, this has begun to give rise to the other people pushing back against the caste system. Yay for internal problems. Similarly, just because its illegal, doesn't mean that some noble houses feed them to their worker caste members.
Well, that was more than I originally thought. Had an idea and my imagination ran with it. Let me know what you all think and tell me if I'm being too specific in my ideas.