Assuming you've found the citadel - you need get past the "waters of life" quest along the main quest line. If you haven't gotten to that point of the main quest, it will come - IIRC it's about halfway or so through the main quest line of the game.

Once you get through that, you'll be able to access the citadel.

I could give you more information or a useful link, but it may spoil some of the story, so I'll only add more to this answer on request.

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