On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough for The Lighthouse Twins side quest.
Rewards: 1,200 XP.
In the place marked on the image above, you will meet a man named Frode. Unfortunately, he has damaged his leg and cannot move, while his brother has climbed to the top of the tower to check out the legend of the Old Viper.
Arne (Frode's brother) is stuck at the top of the tower and doesn't know how to get down. Climb to the very top, unlock the ladder, and eliminate the snakes encountered along the way.
When you're at the top, speak to Arne, and after a short while the man will start coming down the path you prepared. When you are at the bottom, speak with the brothers to complete this task.