As far as the rewards of building and maintaining the castle, I understand the traditional goals of that in a regular civ-like RTS. It sounded like it was being suggested that it be different than that though, particularly in the sense that the user only controlled one guy. Would people follow you? Would additional units just run around with AI and do their own business? Lots of questions are raised in this scenario.
Multiplayer RTS style games would probably not work for Flash because of latency. But when I was describing the RTS I meant that you would be just one unit. You would see the world in third person like most MMOs right now, you would be on a team/faction which could possibly be compared to an FPS deathmatch, but you could gain levels and increase your skills and with the right strategies you could win the game.
It is very complicated and this is just an initial idea, and it does raise a lot of questions, but I think that a proper development team would take the time to answer the questions and make something out of it.