The armoury is where high-quality infantry and low-quality vehicles are created. Its units are more devastating than barracks units, but cost more: indeed, some of the armoury's units cost some oil to be created - roughly about 1 to 2 oil per unit, other than the Supply Truck.
One problem with the Armoury is that you can build one, and only one, within the presence of a Town. This means that in general, the units an Armoury produces are somewhat limited in number and so should be used wisely.
Machine guns - machine gunners are slow, anti-infantry units. While they can massacre hordes of infantry wholesale, they are somewhat vulnerable to cavalry, and are easily destroyed by mechanised units. Has a ramp cost of oil and can be upgraded by major factions.
SMG infantry - the dread of all infantry, sub-machine gunners are armed with powerful, rapid-firing assault weapons. These weapons are sufficiently heavy to take out almost all other infantry, cavalry and some vehicles; however it doesn't have the same range as a rifle, nor can it be used to take apart tanks or battleships. Major factions have the ability to upgrade these units further. Has a ramp cost of oil.
Flamethrower (infantry) - a fearsome infantry unit, specialised in burning units out of holes. Available only to non-Great powers. Costs some oil. Minor powers have the ability to upgrade flamethrowers.