While building their outpost at the end of When Darkness Falls, Hiccup tells Astrid that she can build the most heavily guarded bedroom in Viking history. So she does; complete with a powerful ballista on her roof. Whenever Astrid's hut is seen on screen starting with Gone Gustav Gone, the ballista can clearly be seen on its roof. It's first mentioned however in Edge of Disaster, Part 2, when Astrid tells Tuffnut he can use it to defend the Edge from invading Dragon Hunters. It's mentioned again in Defenders of the Wing, Part 2, when the Dragon Hunters take over the Edge including the ballista, and the riders themselves know how powerful it is. They fear even they would never be able to get past it because they know what an excellent weapon builder Astrid is. But Mala, with Hiccup and Toot