Games with castle sieges:
Guild Wars 2 (am not exactly sure how L2 does it since I never did the castle siege)
Ragnarok (old game but still populated)
Games with realm vs realm:
Regnum (getting a PvP update soon)
Fantasy Earth Zero (foreign server)
Pandora Saga (old)
Gates of Andaron (aka 4Story - old)
Rising Force Online (not sure what happened to this game)
World of Warcraft
Warhammer Age of Reckoning
From the above list, I recommend Regnum (because of the update), Guild Wars 2 (because of the graphics), and World of Warcraft (because of the content).
Ah, Kickstart's lists are more accurate, since the majority of games I put are outdated.