Building an Arsenal requires a Castle or any of its replacements.
With Social Policies / Tenets:
An arsenal is a larger and more extensive armory, containing an army's bigger and more dangerous weapons systems - tanks, artillery, high-explosive ammunition, and so forth. Arsenals are even more heavily-guarded than armories, since nobody wants anybody stealing a tank or an 88-mm explosive shell.
Unique Building Replacements
|Outpost||Cascadia (Osborne Russell)||