The first level is where you'll find the blackrock gem. Use a pickaxe to make your way through the boulders till you get to it. It is also here where you must use Altara's rod. You might also want to pick up the bones that are lying there to give to the ghost Tryestro on the second floor. However, the hints he gives you are pretty much worthless. Anyway also on the first level are several map pieces of level 3. Collect them to make your life alot easier on that level. In the southernmost section of level 2 is Helena, Loth's wife who will ask you to go to level 4 and talk to Praecor Loth. Level 3 is a large labyrinth. However, if you got all the map pieces from level one there should be no problem figuring out what to do and where to go.
Level 4 is the domain of Praecor Loth. To get to him you will need to pass three Liches. The first isn't too difficult to kill. Open the door with the key you found in the secret room on level 3 (near the water to the NE). The Second liche can fly so watch out. After defeating him make your way to the orb on the eastern side of the room by casting Portal (VAS EX YLEM) to get to bypass all the teleportation areas. When you reach the orb on the pentagram, remove the candles and the way will be clear for you to go to the 3rd liche. After defeating it, go east and through the water fall and swamp and pull the chain you'll find. Now you can go to Praecor Loth. He will give you a horn which you'll need to blow to shatter the gem. However if you don't have the air daemon inside you then you will not have enough power to blow the horn.
Go back to Killorn Keep and you'll find now that it is under the rule of Mors Gotha, one of the Guardian's main agents. She has a spellbook which you'll need to give to Nystul so he can cast the proper spell. You can either take the Spellbook and run away or kill Mors Gotha. It doesn't really make a difference though since she won't die (the guardian will come to her rescue). After giving Nystul the spell book he will ask you a couple of questions. If you don't know what the answers are then this would be a good time to talk to Nell. When everything is all set and ready, then make your way to the throne room where everyone will be waiting. The finale will occur at this point.
It was purposely left out the parts when you revisit the castle since this is where the major plot events occur, and so the fun would really be ruined. Besides there's no thinking to be done just talk to everyone every now and then, especially Miranda. For example everytime you finish a new world, it would be a good idea to go back and talk to everyone in the castle. Otherwise certain events will not occur until you do so, thereby preventing you from making progress. Good luck.