The Rift site has been updated with a new developer post to reveal the first information about fortress sieges that will be released with the Starfall Prophecy expansion. Players can literally storm castles as an individual, in a small group or in a full 10 player raid.
Ascended who wish to advance their reputations as heroic fighters (and perhaps line their pockets) will be well employed in keeping the Tuath’de in check – it seems that almost as soon as they fall, more arrive from regions unknown to replace them.
A sole warrior may wish to face challenges around the exterior walls of the fortress, picking off less protected foes, a vital task to support those further in.
Further feats of valor are within the grasp of a group of Ascended, as they penetrate deeper into the concentric rings of fortifications, facing lieutenants with their squadrons.
You can read more on the Rift site.