There are a lot of Troglodytes of varying types scattered through here, so be prepared to fight a lot. You can either go to Ellhioc's corner and complete the quest, or you can clear the entire dungeon, it's up to you.
Make your way through the passages along the eastern edge of the map all the way north until you come to a wall with a door in it. Go through the door, but be aware that there's some Troglodyte Shamans in here who will attack you as soon as you enter. Once you've killed them head through the door to the west and over the bridge.
A short distance past the bridge you'll come to the Barrier which seperates the Troglos from the Giants, or rather the Troglo end of the barrier. There's quite a collection of Troglodytes here, so be prepared. Once you've killed them continue to the west.
The first passage to the north leads to a Drake's nest. Kill the drakes and then check their nests for some coins. The bodies in the corner don't have anything on them.
Go back into the passageway and a little bit further west you'll come across a control panel. Starting at the top you want to push the first button once, the second button twice, the third button three times, the fourth button once, the fifth button four times and leave the last one alone. Now go and look through the portcullis and make sure that the runes are all lined up in a straight line.
Before entering the room with the runes go to the north and you'll see a passage blocked by some stalactites. There's three of them to get through, and you need Move Mountains Lv.2 or a Stoneshatter Crystal to get by. There's a body at the end, but offhand I can't remember what was on it.
Head back to the room with the runes and walk along them to the western wall. You need to use Move Mountains or a Stoneshatter Crystal here to get through the wall. Once you're through you'll be in a graveyard with a door leading to the north. Make sure your party is at full health, and you might want to cast Haste and War Blessing before going through, since Ellhioc is waiting for you on the other side, with his guards.
Once you've killed Ellhioc check the northeastern corner for a secret passage. This passage contains a spell tome that will teach you Control Foes Lv.3.
Now you can either continue exploring this level, or you can return up the ladder to the Troglodyte castle.
If you continue to explore go back to the control panel and then head south. There's a few rooms to the west, none of which contain anything too terribly exciting. The last two contain a Troglodyte Warrior in the first one and a Troglodyte Khazi in the second.
In the far southwestern corner is another Troglodyte Altar, though not as big as the first one. There's a bunch of Khazi's and Shamans standing around the Altar. Once you've killed them you can perform the Ritual of Sanctification on the Altar for 100 experience.
Head east from here, watching for a doorway leading south. This door leads to a room with three tomes. The two end ones are trapped, but the middle one teachs you some Vahnatai Lore.
Just slightly to the northeast of the room with the tomes is another doorway leading to a long hallway. The first three rooms to the east are bedrooms, but only one of them is occupied. At the end of the hallway are some Troglodytes standing around. Once you've killed them check the last bookshelf to the east for a secret passage. This leads to a small treasure room, which is trapped with Quickfire. However, there's quite a few coins and some nice elixirs and brews in there, including a Heroic Brew.
Finish exploring the map, and then head back to the ladder you came down and up to the Troglodyte castle. At this point you can either attack King Vothkaro and clear the Castle, or you can wait until after you've completed the Concealed Tunnel portion of the quest. Once you leave the only way back into the Castle will be through the tunnel joining the Troglodyte castle with the Concealed Tunnel. Note that the barrier blocking the tunnel will not disappear until after you complete the Concealed Tunnel.
If you've decided not to clear the castle now just go out the secret passage to the south in the castle and return to Shamirik to get your reward from Mayor Knight.