Kabaneri of the Iron Fortress

At a time when the industrial revolution was carrying the world into the modern age, a horde of undead monsters suddenly appeared. They could not be killed unless their hearts, which were protected by a layer of iron, were pierced. These undead, which would later be known as "Kabane," exploded in population until they finally overran the world. The people of Hinomoto, built fortresses known as "stations" that have allowed them to survive the threat of the Kabane. Ikoma is a young steamsmith living at Aragane Station, working on developing a unique anti-Kabane weapon called a "piercing gun". (Crunchyroll)