On this page of our guide for Assassin's Creed Valhalla you will find a detailed walkthrough for the mission called Ringing Cyne Belle.
In preparation for the assault, Eivor neutralizes springalds and oil cauldrons at Cyne Belle Castle.
Rewards: 1700 XP.
Speak to Geadric
You will find Geadric in a small camp at the Derelic Shrine of Camulus. Geadric will explain that you cannot yet attack the fortress, because Eadwyn has too many traps and defensive measures prepared. Eivor decides to help and disable the castle's defenses.
Find and weaken the castle defenses
Sneak into the castle and disable 5 traps to fill the mission progress bar to 100%. A single, accurate hit on the weak spot of a device (red point) will get the job done. If you want to perform this task quietly, you can approach the machine and disable it manually.
If you do not want to, you do not have to fight the soldiers stationed in the castle. When the task is done just escape the castle and the guards will not chase you. Go back to Geadric and talk to him to complete this mission. As a reward you will receive 1700 XP.