Sword of the Lictor, chapters 28 to 31
Severian reaches a village on the shore of a lake and uses intimidation to get a good meal. It's drugged. Severian is taken on a boat to a castle at the centre of the lake inhabitated by a giant, who is giving gifts and demanding tribute. The people living on floating islands on the lake suffer for that.
Severian wins free when the islanders attack the boat, along with the islander slave who served him dinner. They make love. Severian agrees to lead the islanders to attack the castle. Part of the reason is that they have the Claw of the Conciliator, sent to them while he was drugged.