Canon Background
https://awoiaf.westeros.org/index.php/Stannis_Baratheon

In-Campaign
In the great hall perched above Gulltown in House Grafton’s great tower-keep Stannis was introduced to the knights of House Jasper (including Jardon) by Nestor Royce. He was humorless and to the point. He told the men that they were commanded to scale the eastern walls of Dragonstone at dawn as he led the main force in storming the gate. He suggested that although a large garrison is not expected that those men who remain would be steadfast Targaryen supporters. Therefore, the Jaspermen were instructed to capture (or kill) the prince and princess so that the castellan would be inclined to yield. He ordered each man to be outfitted with a rope and grapple… and thanked them for Davos’s onions as they parted.