Assuming that you're first approaching the Troglodyte Castle after receiving the quest from Mayor Knight of Shamirik, you'll be stopped at the gates and asked if you have the papers from the mayor. Say that you do and agree to be blindfolded.
When the blindfold is taken off you'll find yourself locked in a small cell. Wait for a while until someone comes and tells you King Vothkaro will see you now. North along the passage outside your cell and you'll be hustled back into the cell by someone saying that King Vothkaro isn't ready to see you yet. However, there will now be a scroll in the cell. Read the scroll and wait some more.
The next time someone comes to tell you King Vothkaro will see you now he actually will. Go through the doors at the end of the hallway and into the throne room. Talk to Vothkaro, and when he asks you to do something for him, agree to do it. You'll find yourself back in the cell, but now the secret door to the west, near the bed, will let you through. Go down the ladder and complete King Vothkaro's quest. When you come back up the ladder someone will be waiting for you with a message. This gives you the quest to bring down the Troglodyte/Giant barrier, so if you wanted you could go kill King Vothkaro and clear the Troglodyte Castle if you want.
Make sure that you don't kill Vothkaro before you get this quest. You don't need to report back to him once it's done, so you can kill him at any time after the quest is given and still get the experience, but if you kill him beforehand you won't get the quest. I don't know what result that would have on the game.
If you decide not to kill Vothkaro now you can leave through a secret passage in the southeast corner of the hallway outside your cell, but once you leave the castle you won't be able to get back in until you've finished the Concealed Tunnel.
If you do decide to finish the castle now just head back up towards the throne room. As soon as the guards see you the battle will be on and everyone in the castle will now be hostile. Go into the throne room and kill King Vothkaro and his guards. Make sure to check the items dropped, because Vothkaro will drop a Ring of Skill.
Now head out the doors to the east of the throne room. This takes you into Vothkaro's bathing chamber, which contains nothing of note. The doors leading north out of the bathing chamber enter into a long hallway running north and then curving west at the end.
The first two doors to the east lead to small libraries. The second library has a section of wall that you can use Move Mountains or a Stoneshatter Crystal on to break through into the next room.
Notice that discolored patch in the middle of the floor? That's a trap, and when you get about halfway across it's going to open up and drop you into a pit of lava.
The door to the north leads to the treasure room. To even open the door you're going to need use Unlock Doors Lv.2 and then Dispel Barrier Lv.2 or a Piericing Crystal, or, if you have it, you could just use Dispel Barrier Lv.3 which acts as Dispel Barrier and Unlock Doors Lv.2.
The treasure room is guarded by three demons. Once you've killed them all check the chests for a nice amount of coins, some Blessed Bolts, a suit of Blessed Studded Armor, and Ghoulbane, which is one of the magically enhanced spears in the game.
Once you're done the treasury head back into the hallway and follow it around the curve to the west. There's a series of bedrooms and barracks along here, and the trash pit in the first room. Don't forget to check chests and dressers for items.
Now go back to the north/south part of the hallway and check the doors to the west. The northern of these two rooms appears to be the meeting room. The second room is where the Troglodytes have set up statues and memorials to their ancestors.
Go back to the throne room now, and through the doors to the south. This takes you to the barracks. To the south of the barracks is one of the sections where Troglodytes watch for enemies approaching the castle. The door at the west end of this passage leads to a small room containing either a Khazi or a Shaman, I'm not sure which. You can also kill the other Khazi or Shaman across the way through the arrow holes with long distance attacks.
Back to the throne room again, and this time head out the last set of doors, which lead west. This is a large courtyard with a lot of the various types of Troglodytes in it. Kill them all and move west again. This is another courtyard, but there's only Troglodytes and Troglodyte Warriors in this one.
The door to the south of this second courtyard leads to another barracks. Again, the barracks have a door leading south to the other section from which Troglodytes watch for enemies. Go check the room at the eastern end to see if the Shaman/Khazi you killed there had anything on him, then head back into the second courtyard.
Go west from the courtyard and you'll find yourself in a small Troglodyte Temple. You can perform the Ritual of Sanctification here as well.
Head south from the temple and you'll be in a hallway which goes in several directions. Straight across should be a storeroom with a variety of items, most of which you probably won't need by now, but check to make sure.
The room west of the storeroom are the Castle's holding cells. One of the cells contains a Giant Cockroach, and one contains a Giant. The rest of the cells are empty.
Go north from here and through the first doorway leading west you come across. This is where the Troglodytes keep a few worgs in some pens. Kill everything in the room and then go back into the hallway.
The next room on the western side is where Troglodytes are taken to be punished or tested. The middle cell contains a Mutant Lizard. They're a lot tougher than Giant Lizards.
The last doorway leading west leads to a smaller hallway with two doors to either side. These are bedrooms. The room in the northwest corner contains the Slow Spellshard in a chest or dresser.
Go through the doors at the north end of the hallway and you'll be in a narrow long area where the Troglodytes apparently grow their crops.
Go back into the hallway and through the doors to the east. This leads to another long hallway.
The door leading to the north is a barracks, and the last dresser to the east contains a Knowledge Brew.
To the south is yet another barracks, this one apparently for the higher ranking Troglodytes. There are also two bedrooms in the northwest corner of this barracks.
Go back into this cross hallway and through the doors to the east. You'll be in yet another hallway, again running north/south. At the south end is the Castle's forge. The doors to the east all lead to the dining hall, and to the east of the dining hall are the kitchens, with a pantry to the south and a storeroom to the north of the kitchens. The pantry contains three cells. The middle one is empty but the others contain a Nephil and a Goblin, both of whom are hostile.
Now head back to your cell and out the secret entrance in the souteast corner. Return to Shamirik to get your reward from Mayor Knight and then head up to Lorelei to begin the Giant portion of the quest.