In order to deliver the Epic Story portion of the game, yet allow new players to come along later, we've decided to divide the Citadel into a spiral set of Tiers. You start at the lowest Tier at the base of the spiral mountain. As you progress your personal story far enough you will gain opportunities to earn your way to the next Tier of the Citadel.
This advancement in Tiers is then tied to the delivery of the next section of the Epic storyline. Thus, you must ascend through the Tiers in order to gain the next 'chapter' of the story. So even if you are further along the story than a new player, that new player will start in Tier 1 where the story begins.
However, we will not lock you within your Tier. If you want to go back down to a lower Tier and adventure with a new player, you may do so. This will not affect your story progression, only slow it down until you return to your current Tier.
Additionally, new areas of the world will be opened up based on your Tier progression, allowing a natural progression of game difficulty with player advancement.