Short answers, pulled from the story.
An arsenal is a place where arms and ammunition are made, maintained and repaired, stored, or issued, in any combination, whether privately or publicly owned. A first-class arsenal may contain a laboratory, gun and carriage factories, small-arms ammunition works, harness, saddlery, tent and powder factories, and large storehouses.
The word arsenal entered English in the 16th century as a loanword tracing back through the Italian arsenale to the Arabic phrase dār aṣ-ṣināʿa, meaning "manufacturing shop." The Italian form arsenale is thought to be a corruption of that Arabic term.
Arsenal and armoury (British English) or armory (American English) are mostly regarded as synonyms, though subtle differences in usage exist between the two terms. A sub-armory is a smaller facility used for temporary storage or carrying of weapons, such as a patrol vehicle operational only at certain times of day.
A floating armoury is a ship that stores weapons to be supplied to merchant vessels in international waters subject to piracy. The arrangement allows weapons to be transferred without entering territorial waters where they would be illegal.
Frederick Taylor introduced command and control techniques to arsenals, including the Watertown Arsenal, a principal center for artillery design and manufacture, and the Frankford Arsenal, a principal center for small arms ammunition design and manufacture. His systematic management methods were applied to both manufacturing and administrative functions within these facilities.
An arsenal usually describes a collection of one hundred or more weapons or firearms. This threshold applies to the general use of the term rather than to the institutional facilities that formally bear the name.