Move up the spiral stairs and you will run into a Hooded Girl. She runs off and Zidane and Blank realize who she really is and chase after her. You will now be in control of Steiner. Head back up the spiral staircase and through the south door. Talk to the Queen and you will receive an item depending on how well you performed during the stage fight between Zidane and Blank.
If you impressed between 1 and 49 nobles she will give you an ether. If you impressed between 49-79 she will give you an elixir. If you impressed between 80 and 99 she will give you a Silk Shirt. If you impressed all 100 nobles you will get a Moonstone.
Once you have the item head out and back down the spiral staircase. Go into the room where Zidane and Blank stole the guard's clothes. Search behind the door to the left to find a Phoenix Down. Talk to Mosh in the corner of the room, he doesn't have any mail for you to deliver. You can save your game here if you feel the need to. You can explore the castle now if you like. Once you are finished go to the room with Brahne's picture and head down the stairs. Move through the room and leave through the room from the bottom.
You will find yourself outside, move to through this area and go into the next area from the left side of the screen. Go into the opening and move up the spiral staircase. Steiner will spot Zidane and Garnet. You will then gain control over Zidane once again, follow Garnet when she runs through the door. Follow her down the stairs and watch the scene.
Once you are in the engine room turn the wheel on the large machine. Move over to the stairs and move to the lower level. Open the chest that fell to receive a Phoenix Down and a potion. Once you are finished move back up the stairs and leave the room.
As you walk through the next room will have to battle Steiner.
Steiner is extremely easy, since he is one of your first battles. You will have to battle Steiner three different times. The first time you encounter him is in the Prima Vista after you have found Garnet. Just attack him normally and don't forget to steal.
Have Zidane and Blank dealing the damage and Cinna doing the stealing because his attacks are weaker. Make sure that you have stolen the Leather Hat and Silk Shirt before you end the battle.
The next battle is basically the same thing except now you have Vivi, Garnet Zidane and Marcus. You also lack the Steal Skill, so I hope you stole those items when you had the chance. Use Vivi's Fire skill and press L1 to select all of the enemies to divide the damage between the three of them. The guards will run off once they have taken enough damage. Just continue attacking Steiner any way you like until the battle ends.
For the third battle you have the same party as before. Attack him as you did for the last one and at the end of the battle Steiner will notice the bomb behind him.
Once the battle is over, watch the scenes that take place and get ready for the next area.