To those that don't know me, I'm one of the more experienced multiplayers and in the absence of a general guide here to multiplaying, I'll add some suggestions to try and help people get started with online play for BI and in the hope of encouraging some discussion. The contents of this post though isn't intended for more veteran players, but any comments are of course welcome.
Briefly, about denarii. Most experienced players play at denarii levels around 12.5k, so this is the level I'm addressing. This is not to say that other levels are bad, though playing at much higher levels does imbalance the game, as the balance as it is is based on powerful units being expensive, so few can be taken thereby weakening the rest of your army. If this limit is removed, then there is little point in taking anything but the elite units. Much lower denarii levels are also regarded to be a little too much the case of archer festivals, since cheap infantry defends poorly against them, so much damage can be caused. It's for this reason most tournaments have the level set to 12.5k as a compromise level with a flat map so as to avoid camping and provide as fair a battlefield to both sides as possible. For friendly games this isn't so important though.
In BI, most army builds tend to be heavier on infantry and lighter on cavalry as opposed to Rome, which is probably an easier thing to start out using.
Many factions in the game are basically copies of another faction. West Rome/West Rome Rebels and East Rome/East Rome Rebels being the most obvious one. Other identical factions are the Vandals and Slavs, the Goths and Ostrogoths, the Sarmatians and Roxolani the Burgundii and the Lombardi, with the Allemanni being almost identical except they do not possess the "Horde" units.
As I'm not sure the simplest way of writing a guide, so I'll talk through one of my setups and how it is used.
One of my basic 1v1 setups for the Burgundii/Lombardi/Allemanni against a similar kind of faction could include a line of 6 archers, 6 chosen axes 4 spears and 4 heavy cavalry. The archers are often a very useful force for winning a game, as what often will be the case is the person who loses the archer battle is then forced to attack and will often lose the game. So for the reason it is pretty usual for people to spend fair amounts of money on archers, both on the units and on upgrades.
An important note about upgrading archers is that valour upgrades effect the units attack and defensive skill, but for incoming missile fire, only shield and armour apply for defense, so valour upgrades are generally a bad idea in place of weapon and armour upgrades.
It is generally a good idea to keep varying the exact make up for the army, as if people know what to expect from you, it is much easier to counter. So for this reason, for this setup with archers for example, I'll often vary between weapon 3 armour 3 Horde Hunters and up to Lombard archers, perhaps as much as weapon 2 armour 2. As for the type of archer as Lombardi, this depends a lot on the amount of money you wish to spend, and the type of game you're playing. For team games, it is often important to have a range advantage over your opponent so that they are forced to initiate the missile duel, so you therefore get a couple of free volleys of fire in. For 1v1s this is still important, but slightly less so since it is slightly easier to pull off a co-ordinated rush.
The chosen axes usually have their weapons upgraded, since they are very powerful shock units that can often rout entire lines in seconds. They are very vulnerable to archer fire and flanking moves however, so need to be protected. Due to the low morale on the spears, I often put a valour upgrade or two on them, so that they hold for longer. I rarely upgrade cavalry in BI since it already is pretty expensive (though it can be an idea for surprise).
As for using this setup, I would generally setup with archers in loose formation spread out in front, the chosen axes behind, but with a gap so that they don't take stray fire from enemy archers, 2 spears on either flank and cavalry at the back. Often I use a cavalry unit as a general for this faction, as most of the rest of my army can be quite easy to rout under pressure, and it means the general can be easily put in and taken out of the fight. A little tip for a nice layout is to select the units in order of how you wish them to appear in a line, i.e. unit, unit, general, unit, unit and then right drag click to set the position and group. This just sets up the group so the general is safe in the middle.
For beginning the battle, moving the entire army at once is easiest by pressing ctrl-A and then holding down Alt while right clicking. This keeps everything in formation while moving. This shouldn't be used in proximity to the enemy though. I tend to move forward slowly, while constantly keeping the cursor over the enemy and in front so that I can gauge the archer range, so be able to stop at a good place. If you have range superiority, it is simple to start, with inferiority you just have to rush in. I prefer using my archers in one group and then double right clicking so that the archers automatically target units and start running, and while still running manually target the archers if there is infantry in front. The easiest way of doing this I find is to select one archer, double right click and then tab to the next archer and repeat.
A common tactic as mentioned is to have well armoured and shield infantry, such as the Roman Plumbatarii out in front of the archers, since they are practically impervious to archer fire head on, so while archers are firing at them, you are getting in free hits on their archers. This is easy enough to counter by manual targetting, but it does pretty much prevent skirmishing of archers by cavalry, which can be useful later on in the archer duel.
As for the targeting of archers, usually I focus down units individually to a small number before moving onto the next, so therefore enemy archers will be firing their arrows at their full size rate, so exhaust their supply quicker. I often will have 2 or 3 units focusing at one archer, until it gets down to about 10 in normal scale before moving onto the next. It's important to not get too obsessed with wiping units off the field when there are other more interesting targets. If you win the archer battle, then start focusing units such as cavalry or chosen axes that you can do damage to, and if possible try to get a bit of an angle onto enemy units so that their shields do not come into play. They will likely attack then, but if they do not, keep firing until you run out or they do. If you have lost (or preferably about to lose the archer battle, attack, chosen axes are very vulnerable to archer fire, so you don't want to lose too many, though this is not always the case in team games, where it is more important to move together, even if you lose most of your army to archers).
If you are defending, then just sit tight until they get close. If they try to flank you, keep rotating (easiest accomplished by pressing ctrl-A and Alt-right drag clicking on your army) so that you face the main body of their army. If they spilt up, you'll need to divide a little more, but don't get too separated. This can be an advantage to seize by attacking suddenly. Remember to warcry as they get close, and charge in at the last moment with the Chosen Axes. For the actual attack, I use the units in groups with double right click to get them moving and then tab through the units individually to make sure they attack the right units. The spears are there to help protect the flanks of the chosen axes against cavalry; a well positioned shiltrom can hold enormous amounts of cavalry back. Your cavalry should basically go against their cavalry for this setup, as your chosen axes are the winning force, but against other chosen axes, charging into their flanks when engaged will pretty much rout chosen axes on contact.
Against Roman infantry, it is important to counteract pila and attack ASAP when they get into range. It is generally a very good idea to also have screening troops in front of your main army, so that they take the pila fire instead. Any remaining archers do this job quite well, and units in a shield wall seem to be the best for actually absorbing pila fire.
For cavalry factions, i.e. against horse archers, I would take more cavalry, as your infantry cannot really deal with the them. Horse archers can pose problems in capable hands.
Well I hope this is of some use to someone, and of course any comments are welcome.
