This specific quest can be completed only if you decide to help Iorveth instead of Vernon Roche, when you are asked to side with one of them.
The Floating Prison wont be available if you decide to ally with Roche, and to complete At a Crossroads: Vernon Roche instead of At a Crossroads: Scoiatael.
If you ally with Roche, The Floating Prison quest is replaced with Death to the Traitor!
The Floating Prison is activated after you meet Zoltan in the elven ruins, and you promise to help Iorveth (actions explained in our previous walkthrough).
“Geralt was more eager for vengeance than ever. Above all else he wanted to find Triss, for he still believed the sorceress to be alive.
He was not picky about the means to his end, and it all began with a meeting with the Scoiatael
After you talk to Zoltans dwarves and Iorveths elves, deep in the southern forests, you must find a way to help Iorveth.
Assuming that you start this quest in the ruins, you have to travel west until you find Zoltan along with a group of Scoiatael elves.
Zoltan will ask you to go around the city walls with a group of elves. Stay alert because you will encounter many guards.
Combat Tip: Igni Sign can be very useful against the Flotsams guards. The elves will fight by your side; therefore you wont have any problem to reach your destination.
When you get near the harbor, you will see Roches boat leaving.
Now, you must find Iorveth.
Using bombs and Geralts Signs fight your way to the barge where Iorveth located. You will find him on the deck. Clear the harbor and free him.
The next cutscene shows Loredo on a balcony yelling at you that he will kill the elven women if you sail away. Geralt wont allow this to happen, so you go back and pursue Loredo.
You can also fight your way to the tower, if you wish, and free the prisoners. In this case, make sure that when you reach the top floor you mash the button that appears on the screen (usually A) as fast as you can, or you will burn. A cutscene starts when the elves are free, and you will end up on Iorveths boat.
If you decide to take Loredo out, chase him on the docks, and ignore the burning tower. When you catch him (near the scaffold in the town-center) kill his guards, then using the Yrden Sign or the Igni Sign, stun him. Continue to hit him until he falls to the ground.
Make sure you loot the items he droops, including the Royal Mail Key which allows you to open the box in his office (for the Mystic River side-quest). If you go to Loredos mansion, in his office is a box containing a letter to Dethmold. Make sure you pick it up, if you missed it during your first visit to the residence (Indecent Proposal quest)
Now, return to the docks and use the river boat you find there, to get to Iorveths ship.
The chapter ends with a dialogue between you and Iorveth, who tells you that Dandelion was hurt. If you ask him about Roche, a cutscene starts and you will play as Iorveth.
Hit Roche several times, and the short scene ends with Roche promising that he will find you (Iorveth). Along with Iorveth you sail away towards Aedirn. Chapter 2 (following Iorveths Path) starts now.