The Siege of Vergen

The Siege of Vergen is the main quest that closes Chapter 2 of The Witcher 2: Assassins of Kings on the Roche path. After Conspiracy Theory, Roche is in a murderous mood and ready to take on every Kaedweni in sight. Henselt’s army is storming Vergen, and since the main gate is out of reach, Geralt talks him into using the secret tunnels beneath the town instead, reached through the forest ravines.

Along the way there are Kaedweni patrols, a pair of trolls whose mood depends on Geralt’s earlier choices, and, in the tunnels, Adam Pangratt’s mercenaries and the sorcerer Dethmold himself. Inside the burning town Zoltan points Geralt toward Philippa Eilhart’s house, where Síle de Tansarville is hiding. Geralt can head straight there or first help Iorveth, who is holding out at the town hall.

Síle teleports away before Geralt can grab her, and then Henselt turns up in person with his officers. Once the king is beaten, Roche wants his head, and Geralt decides whether to let him have it. Afterwards Geralt and Roche set off for Loc Muinne.

Walkthrough

Find Roche at the cantina and suggest using the secret tunnels under Vergen.
Travel through the forest ravines, fighting off Kaedweni patrols. Deal with the female troll and then help the male troll fighting soldiers near the tunnel.
Enter the dark tunnels (a Cat potion helps), cut down Adam Pangratt’s mercenaries and lure exploding rotfiends into their groups.
Fight Dethmold, Adam Pangratt and their men. Dethmold retreats behind a barrier at low health; then spare or kill Adam and take the silver sword Nevde Seidhe.
Leave the tunnels into the battle, talk to Zoltan, and optionally help Iorveth fight off the Kaedweni at the town hall.
Head to Philippa Eilhart’s house, beating a golem on the way (Yrden, dodge its charges); Síle escapes.
Fight Henselt’s ambush: two two-handed swordsmen first, then Henselt with two shield-bearers. Keep Quen up and use Yrden.
Decide whether Roche kills Henselt, then watch the cutscene as the chapter ends.

Quest Consequences & Choices

Help Iorveth or not: helping him at the town hall has little effect on the story, though he speaks bitterly of his situation afterwards.
Henselt’s fate: stop Roche and Henselt lives; let Roche kill him and in Chapter 3 Dethmold leads the Kaedweni forces in his place.
Adam Pangratt: sparing or killing him does not affect later events.
Trolls: if you spared the female troll, the male distracts the soldiers at the gate; if you admit to killing her, he attacks you.