Cask was a natural fortress, sitting in a high-walled canyon that provided a natural bottleneck. Cask was also protected by a massive arch of rock, which protected it from air attacks.
In 2503, large numbers of zerg invaded Sorona. In response, the colonists fortified the bottleneck, calling it "the Wedge". The defenses had paristeel gates added to them. The zerg fruitlessly attacked daily. The colonists continually called the Dominion for help, but the government did not respond.
After eight months of conflict, the zerg forces had dwindled to a relatively small number of zerglings when the Dominion Marine Corps 128th Platoon, aka "Iron Jesters", arrived. They had been dropped off as excess passengers as their battlecruiser traveled to Korhal for refit, and they were unprepared for combat. However, three quarters of the platoon made it to Cask alive and joined the local defenders.
After six months of holding off the zerg, Private Maren Ayers, a scientifically-trained medic, discerned the purpose behind the seemingly pointless zerg attacks – according to a theory of her father's, when a population shrinks in size, its mutation rate increases. Ayers autopsied some dead zerglings and discovered acidic green pustules, derived from a local mold. When she tried to dissect one, it exploded, injuring her.
Ayers' warning came nearly too late. The new breed – banelings – rushed into Cask as Ayers contacted the Dominion with this new intelligence. Four days later, a Dominion fleet finally arrived, too late to save the colonists of Cask and the marine platoon.