Also, the Armory allows training of Military Engineers - a very important support-class unit. Do not underestimate the importance of this, because this unit may be extremely helpful in various situations in the game!
An armory (or arsenal, the terms tend to get mixed although there are some significant differences) is the place where arms and armor are manufactured, repaired and/or stored when not being used to kill other people – i.e., during peacetime. Unless planning on facing the enemy, soldiers usually do not carry their weapons and armor about with them (especially if their weapons were lances and their armor plate). The first armories date back to the first professional armies, those of Sumer and Akkad. Every standing army since has had armories, perhaps as simple as a well-guarded (wouldn’t do to let the commoners get hold of any weapons) warehouse or as elaborate as a production facility making muskets. Given that it didn’t make sense to have the weapons on the other side of the city from the troops, the armory was usually near the barracks. Armories have even shaped the course of civilization, for a variety of reasons: the Tower of London, Harper’s Ferry, the Kremlin Armoury, Springfield Armory, the Zeughaus in Berlin, and so forth.