Talk to Lehtinan and go through his conversation menus until you convince him that you want to go into the back rooms. Head back to the bouncer and go through the door that is now unblocked. Talk to the guards if you wish, but head through the door leading to the northeast. You should find yourself in a long passage leading to the slave chambers. There are a couple of secret rooms on the north passage that your thieves might spot. The one on the right leads to a sewer maze, so don't examine it yet. Keep heading to the northeast until you reach the prison.
 Once there, you'll be attacked right off by a few guards. A few of them appear to be spellcasters, so be cautious. Once they're dispatched, talk to Hendak in the middle cell. He'll ask you to free him and the other slaves. You'll need the beastmaster's key for this.
Leave the slave cells by the southern door and head to the south across the gladiator pits. You'll find the animal cells to the southeast. Once you are hostile to the beastmaster, he'll unleash the animals in the cages, so be careful. There are a few nasties in here, including a black bear and a minotaur. If you manage to kill the beastmaster quickly, you can try to dart up to his corpse, take the key and the rest of his loot, and escape the area before the minotaur notices you. Either way, get the key and head back to the slaves. Release the kids on either side of Hendak before you open his cell; they'll each nab you 2,500 experience points. Once Hendak is released, follow him back down the hallway, dispatching guards as you go. If you can follow him quickly enough, you can force-attack Lehtinan to make the fight easier for him (though Lehtinan is doomed either way). Once Lehtinan falls, the Copper Coronet is under new management, and you can speak to Hendak for a new quest involving slavers working in the city. Don't forget to search Lehtinan's body for a large amount of gold and some other goodies.
If you wish, you can visit the brothel at this point or speak to Bernard to sell any items you don't want. Once you're ready to head out and kill some slaverunners, head back to the secret passage and go up the stairs to the sewers.
 There is a secret stairwell near the gladiator prison which will lead to the sewers. Pass through the sewers to the Slavers Stockade.
You'll be ambushed by hobgoblins right off. These guys are fairly tough, so swarm the shamans first. Once they're dead, search the bodies, then head back to the branching corridor. An otyugh will appear with a couple of slimes nearby. Kill them all, then look at the grate in the middle of the room. Reach down to pull out a hand from the sewer.
 Head back past the hobgoblin corpses and into the cave area. There's a branch to the northeast heading to a room with three myconids. It's not necessary to kill them, and be aware that they can confuse your characters. Search the skeletons on the wall in the cave for a ring.
 Follow the southern path and eventually reach more sewers. The door that lies across the pit is trapped, so be sure to detect and disable it. Look around to the south if you want. To the north is a group of kobolds. They'll threaten you when you appear, but won't go immediately hostile. You can take them out with a fireball or similar spell while they are waiting around or just hack and slash through them. One of the shamans may cast a large entangle spell on your party, so be wary. You can nab a shaman's staff from the dead shaman; it can cast goodberries once per day, but will also be needed for an upcoming puzzle.
 Keep heading south. The path will veer to the west and east. The western path is trapped, and the eastern path has another party of hobgoblins to deal with. Meet Quallo in the south, and he'll tell you to find the "clues." Head to the northeast until you find the room with the pipes; a disembodied voice will ask you to free it from the water. You can click on each pipe to get a hint as to what item you must use on it. Head back to Quallo and kill his carrion crawler pal. Pick up the blood and ask him about each item. The riddles are fairly easy to solve, but if you need help, here's the order in which you should place the items: hand, ring, blood, staff. Going from left to right on the pipes, click on them in this order: third, first, second, fourth. Each party members will gain 18,000 experience points, and you will find a nice new intelligent two-handed sword. Head to the south and up the stairs to the slaver's ship. You might want to rest up first, as you'll be facing a bit of a battle.
 Once you arrive, Captain Haegan and his men will begin assaulting you. Haegan is a tough one, and there is a priest that will cast defensive spells on the rest of the men as well. Eliminate them and search the bodies for items and especially don't miss Haegan's body.
 Use the key to open the door in the northeast wall, defeat the Trolls, and free the girl.
 You should position your warriors near the door and lure the trolls out, one at a time if possible. Hack and slash away until they fall down, then have a character cast a fire or acid spell on their bodies (Melf's acid arrow works well). If you don't burn their bodies, they will regenerate and fight again. They both offer 1,400 experience points. 
Give the girl nearby 100 gold for an easy reputation point. 
Now, head through the northern passage in the middle of the ship and free the two children for more experience. 
Head into the doorway to the northern room and detect the traps. 
There are a couple of yuan-ti in the nearby room; kill them and detect the traps here, too. 
Search the rooms and get the items. Heal up and get ready for another tough battle.
 To the southwest, there's a room with a couple of wizards as well as a guard and an archer. A silence 15-foot radius spell is a good idea while the wizards are grouped together or perhaps a Fireball. Move your tougher characters through the doorway and start taking out the spellcasters, then eliminate the other foes. Do a detect traps on the chest near the door as well as on the stairs leading down and search the area. Take a look around the area to the south (the shelves are trapped) and head out the door nearby.
 Return to Hendak in the Copper Coronet and he will give you a reward from the spoils he found from the slavers.