To be able to access the area around The Arsenal, part of Tatara Tales must be completed so that the Mikage Furnace area can be unlocked; before then, the Containment Field will prevent players from entering. The Arsenal itself can be entered after obtaining 3 Keys to Some Place that are found around Tatarasuna in Common Chests. Upon first entering The Arsenal, three Ruin Sentinels will spawn.
In The Arsenal, there is a Luxurious Chest that contains 30 Primogems, 30,000 Mora, 3 Hero's Wit and the Diagram: Katsuragikiri Nagamasa, which is used to unlock the forging recipe for the Katsuragikiri Nagamasa. Obtaining the diagram grants a Wonders of the World achievement, "Blade of Tatara". Additionally, there is 1 Electroculus, a number of Naku Weeds, and an Enigmatic Page.