With this week’s update we are adding the option to join a Castle Siege while the session is in progress. We are also introducing new updates improving the multiplayer. Please bear in mind that this is the first iteration and there might be some issues (mainly related to clients' connection on dedicated server).
https://www.youtube.com/watch?v=ZsazTt8L0UI
Full list of new features and fixes: http://forum.keenswh.com/threads/7372139