If you are still in the act 1, but have not reached the citadel yet, just keep going and you will get back to the first level eventually.
If you are in the citadel, <span class='standouttext'>Spoiler : </span><span class='spoiler'>return to the start of the citadel where you killed the guard and had the option of wearing the citadel guard armour as a disguise, then take those stairs down to the first level (into the room a guard blocked near the start of the game). Then just head west, opening the door with a lever if you had not done so previously.</span>
