The Armoury is where your weapons produced by the weapons workshops are stored. They are then used to create military units at the Barracks. In Stronghold 2 the armoury has unlimited storage capacity.
Stone Required - 5
Wood Required - 5
Many of your Military Units are created and trained here, you need gold, available peasants and for all but Armed Peasants you will need to have weapons and sometimes armour in the Armoury.
Stone Required - 10
You can buy Mercenary Military Units here. They are more expensive than units from the Barracks but you are not required to provide their weapons. You do need to have peasants available.
Wood Required - 15
*) denotes a former staff member.