Since I have seen no strategy articles covering armies yet, I’ve decided to come up with one, hopefully solving some of the misunderstandings about armies. This guide is for Conquests only, so many of these things don’t count for vanilla/PTW.
What I am going through are the properties of armies and a few known bugs. Much of these info were gathered from different threads so thanks to those people. 
This is mainly a newbie guide so some of it might seem familiar to alot of people. This article doesn’t cover how to use the armies effectively — only the basic properties and the bugs about them.
What are armies?
Armies are created by great leaders to create the army in any friendly city. They can also be created in a city containing the Military Academy (a small wonder). Once the army is created, other units can be loaded into the army. All the units in the army move around on the map as one unit together. The units then share hitpoints as if they were one unit.
The properties of armies:
- The attack and defense of the army is a tricky part. The formula for this is:
Attack = A+(TA/N)
whereA is the attack of the unit currently doing the fighting.
TA is the attacks of all the units added together.
N is 4 if you own the military academy, else just 6.
This formula is rounding fractions down. To find the defense just use this and replace attack with defense (pretty obvious). Note that when right clicking an army it doesn’t show it’s attack and defense as it really is, only the unmodified attack and defense.
- Armies have a capacity of 3 units although the Pentagon increases this to four.
- The armies movement is equal to the movement of the army’s slowest unit +1. So an army consisting of three knights would have 3 movement points.
- Only combat units (those with an attack and defense) can be loaded into an army.
- When defending, an army gains defensive terrain bonuses as a percentage of the improved defend value, not the value showing when right clicking it.
- When no units have yet loaded into the army, the army will only have a movement of one. Furthermore the army’s movement along roads is reduced by 1. So following a normal road an army will only have two moves, on a RoR road (which gives a 4x road multiplier) the movement for an army would be 3.
- You must have at least four cities for every army you create. If you lose cities so there is no longer at least four per army you won’t lose any armies, you just won’t be able to create any more before enough cities are built/captured.
- Armies have the blitz ability which allows them to attack multiple times in a turn.
- They have the radar ability which allows them to see two squares regardless of terrain.
- Armies can pillage without using movement points.
- Armies heal at the rate of one HP per unit in the army. This can also be done in enemy territory even before battlefield medicine. Furthermore armies are also able to heal in one turn in a city containing barracks.
- When transporting armies in ships each unit in the army plus the army unit itself requires one free space (so a normal 3 unit army requires a transport with capacity 4).
- The army unit itself as well as units in the army requires support every turn.
- Units in armies can receive promotions just as normal units. The downside is that only one of the units in an army can get experience (I’m not completely sure here). Also armies cannot get automatic blitz promotion . It has to be gained through many combats (unless you are lucky, off course). Also the army unit itself cannot get promotions only units in the army.
- Units in armies cannot be upgraded.
- An army only gains a special ability if all of the units in the army have that ability (e.g. an army of 2 marines and one rifle cannot make amphibious attacks).
- An army can never gain a worker function even if all of the units in it have that worker function (So a crusader army cannot build fortresses).
- Armies cannot make any MGL’s, not even if the army unit itself is elite status (not possible in the game but I tested it through the editor).
Known bugs about armies:
- The AI will almost never use their leaders to create armies (in fact I’ve never seen or heard of an AI creating an army in the Conquests expansion).
- When they have an army (like in RoR) they will usually put a stupid combination of units in it (e.g. i once saw Carthaginians make an army of two elephants and one swordsman in RoR).
- AI’s are very hesitant to attack your armies. Since the AI calculates everything on a unit-to-unit basis it will have a very low chance of winning with just one unit, and since the AI doesn’t know combined arms strategy they will almost never attack your armies. The few cases where some will probably see an AI attacking an army is when the AI has an army of it’s own, if you got a very weak army, or if the army defends a town that the AI absolutely must have.
That was all their was about the basic properties. Hope this helped solving some of the misunderstanding there has been about armies. If you have any comments please don’t hesitate to share them.