Hey hi! When I saw your forum name I thought maybe you had posted in the wrong forum section till I read what you said
I can't speak for anyone else but for me, ROSE is still as much fun today as it was when I first started playing (in 2007). Lots of changes so you should be prepared to feel like a complete noob but don't worry, soon you will feel like you never left the game. Certainly it's not as crowded as it was years ago but there are still many of us enjoying the game.
The servers were merged in 2015 so there is only one choice when you log in. Muses are still important but not the easiest to level solo. Many people dual client and make both a muse and a fighter (like soldier) so that might be something to consider. Not sure what you mean by "castle sieges" but if you are talking about Union Wars, those have been gone for awhile now. We have valor and honor now. Valor is pvm and Honor is pvp and the points you earn can be spent in the respective shops.
We have a ROSE Discord chat. Here is the info on that. Send me a friend request (HB#4216) and I will be glad to help you get caught up on the changes since you last played.
Looking forward to meeting you!