The primary consideration in any mass-player event is going to be lag. The more Free-for-all the action is allowed to be, the more crowding and the more lag. Perhaps, this won\'t be as much of an issue by the time all the levels \"inside\" are finished and something like seiges and player towns are considered for the surface...but only perhaps.
As has been stated eleswhere in this thread, an actual seige took a very long time and had little \"action\" going on that would keep players occupied or offer much enjoyment. The only part that really makes for a fun game experience is the final assault.
Forgetting about Player vs Player city assaults for now, let\'s just look at what a PvE \"seige\" could be. Firstly, I see them as the last part of a series of linked quests. These quests would be accepted and undertaken by either one guild or a group of guilds with teams within one guild or individual guilds completing different quests such as assembling materials, building and postioning seige weapons, protecting them, \"recruiting\" troops, scouting, spying, etc. This could take days or even a week or more to complete.
Once all the preliminary quests were completed, the guild(s) would be instructed to assemble and begin the final assault. The \"prep\" work and intitial parts of the seige having been carried out in the pre-quests. Each guild, if many were involved, or guild leader, if one was involved, would be assigned particular tasks by the quest giver. These might be on a menu where the guild leader could assign individual teams to each task or, with multiple guilds involved, chosen from a list the quest npc shared with the guild leaders. These would be the actual attack orders: take the South Gate etc and would also \"progress\" or advance and change as each goal was achieved.
Upon final victory, Honor or Bravery points would be distributed, the npc leaders would be executed or exiled, spoils would be shared and everyone goes home....until the next time. It would also be possible to stage these \"seiges\" in several places with greater and greater prep required and difficulty in the final assualt...and provide lots of valid RP possiblilites.
One big advantage to this is that it spreads the action around thus reducing lag. Another is that it allows many small \"rewards\" to be built into the whole, larger scheme. A third is that it would allow for quests of varying difficulty to be included so lower level players could participate and not just the very high level.
Additionally, no players hard earned town is destroyed. I frankly consider that sort of activity (player town vs player town seiges) to be so destructive as to be a really good reason for people to stop playing any game. Not to mention that the winners soon become an unstoppable juggernaut. Shadowbane being a perfect example. If you want to have big PvP battles, have them in open ground somewhere even up to and including the building of temporary palisades and whatever other mechana of war you want to employ.
That looks like more than my two cents worth but since it\'s an open source project, you get a discount!
