This quest begins immediately after Fiery Ambush. It’s time to catch up with Geadric and proceed with the plans for a direct assault on Eadwyn’s forces. You’ll find Geadric at the Derelict Shrine of Camulus, just east of Eatun Barn, or south of Buckingham, where Geadric has set up camp.
Once you reach the camp, you’ll find Geadric in the centre of the temple ruins. Speak with him to learn a bit about his plan of attack. Eivor will volunteer to infiltrate Cyne Belle Castle and sabotage the springalds and oil cauldrons there. You’ll find the castle southeast of the camp, as shown below.
When you reach the castle walls, climb them from the northwestern side. Here you’ll find a battle tower which you can ascend, killing the archer at the top. From here, you can move through the castle using the wooden platforms and ropes to the southeast. Use Odin’s Sight to highlight the guards and locate any nearby oil cauldrons and springalds. The latter are essentially ballistas which fire powerful arrows. You’ve encountered them before during Pilgrimage to St. Albanes. If you want to remain undetected, you’ll have to be constantly vigilant of guards, mostly archers, which can spot you from a great distance. Take them out as soon as you get the warning that you’ve been seen.
You will find cauldrons and springalds throughout the castle, so it’s just a matter of staying out of sight, using Odin’s Sight to highlight them, and sabotaging them as you go. If you do get caught and you’re overwhelmed by soldiers, just run from the castle until you lose them, or hide in a haystack. When the progress bar is completely filled, you’ve sabotaged enough and can return to Geadric. To sabotage the cauldrons and springalds, just get close to them and interact with it. If you want to cause an explosion to damage any nearby guards that might be chasing you, shoot the links on either side of the cauldron. You don’t want to do this unless absolutely necessary, however, as it’ll alert all the guards in the area to your position.
Once you’ve filled the progress bar, you can return to Geadric at the Derelict Shrine of Camulus to complete the quest. If you want to destroy all the cauldrons and springalds that you can find, even beyond the progress bar, this will make the assault on the castle a bit easier during the next quest, but it isn’t necessary. You will have the option to assault the castle immediately, or leave it for now if you have other things to do first.