Could someone explain what is the idea of the schiltron formation (historically)? Also, what is the best use for it? Is it better overall than just the basic "box" formation that is the default.
Also, has someone noticed how formation shape affects archers and crossbowmen? Any research on effectiveness of different shapes?
I tend to make my archer units into lines of box shaped formations
Use it against cavalry and behind gates etc, does work against other infantry, but movement is only at walking speed so in a field battle it may not be advantageous.
The morale seems to hold a lot better in this formation as a spear militia unit can decimate a generals unit even taking disproportionate casualties.
Historically, schiltron is a generic term that's been used to describe units of Scotsmen (I think it's culturally tied to the Scots and only them, I could be wrong there) with big long pikes forming a wall of spearheads, in a variety of formations, either circle, rectangular, square...
Ingame, the schiltron (or hedgehog) formation is more specific, meaning spearmen fighting back-to-back in a circle, with their shields and spears facing outwards.
It has its pros and cons compared to the regular box :
On the plus side, it doesn't have neither flank nor back, attacking a schiltron from any side is the same (both for the attacker and defender) both combat-wise and morale-wise, which means it's a good move to use when you're surrounded or flanked (or are about to be). It's also quite resilient against cav, who'll take heavy losses from all those spearpoints. It's also a very compact and dense formation, meaning that should a wide cav unit charge it, most of the horsemen will just whizz by going "Uh ?!", then stop and wrap around it, but now they've lost both momentum and charge bonus and they've become spear fodder. Oh, and additionally, as of now, it seems to hold formation much better than the box which is a great plus to spearmen. Right now boxes tend to space out more and more as the fight goes on, with units losing cohesion and so forth. That doesn't happen in schiltron, but *crossed fingers* it'll soon be patched
On the con side, it doesn't have any ranks, so no more morale bonus for additional ranks, and no more spear attack from the second rank (and third and fourth for pikes, but those can't go schiltron can they ?). It's also very very slow on the kill (as if spears didn't have enough trouble killing people already

) because the men's first priority is to hold formation and keep their shields up at all times. And, as chunkynut said, it's slow as mollasses (and tiring, too) on the move.
I don't use it much myself, as it seems to be more of a desperate, last stand measure than anything, but from what I gather, some folks here have had good experiences with schiltrons holding bridges and gates. It's very situationnal though - better than box in some cases, way worse in others.
As to archers (and crossbows), I'm not sure if they've kept that from STW/MTW or if it's handled differently in the Rome engine, but it used to be that on flat, even ground, only the first two ranks can fire properly, any additional rank can't see what they're shooting at and just goes ballistic - big accuracy penalty. Same penalty applies if the archers are sitting behind another unit, because them spearmen ain't made out of glass, dontcherknow, so they can't see, plus they have to fire in an arc instead of straight so the arrows are less deadly. In MTW/STW, putting crossbows behind other units on flat ground and tell them to shoot through resulted in many, many pierced backs because crossbows can only fire straight. I hear they can fire in an arc too in M2TW, but lose most of their power. Haven't tried that yet though.
Anyway, bottom line is : on flat ground, the best formation was 2 or 2,5 ranks, and placing them in front of your army.
But things are different when on a slope : if the slope is steep enough that the men on the third rank have their heads above those on the second rank, then they can fire without any penalty. Same goes for following ranks of course.
In that case, it was better to use deeper formations, because then fire will be that much more concentrated, more casualties in a smaller zone, big morale drop for the targets etc... plus you can also put them behind your battle line because they'll sit above the spear line and the range bonus for higher ground will hopefully compensate for what range you lose by putting them behind. Oh, and while your battle line probably has efficient shields, archers usually don't (except the pavise crossbowmen) so letting the regular grunts deal with enemy retaliation fire is a good idea :)
Horse-archers are a different breed though : while their shooting follows the same rules as footmen's, most of the time it's best to deploy them in perfect squares, that way they can skirmish just as easily and quickly in any direction, even though they won't shoot as well.
That's the way I've been using my archers in M2TW too, and it's worked for me thus far. The AI also seems to be following these rough principles.
Oh, and gunpowder units were a different story too : I haven't played with them yet, but back then you had to put them in 3 ranks, no more, no less, and obviously THEY can't fire in an arc (I hope). The 3 ranks thing has to do with how they fire in a cycle : first rank kneels and fires, then runs to the back of the unit, starts their looooong reloading process while slowly walking forward, and is ready to fire again when they're back on first rank. So if you deployed them in 2 ranks, they couldn't do a full cycle and there was a much longer delay between volleys, while in 4+ ranks the additional ranks didn't take part in the cycle and never got to firing. I hear in M2TW they sometimes "hang" mid-cycle because of clipping issues though, with the men running back bumping endlessly against those who want to move forward and shoot. Again, that's hopefully going to be patched eventually.
Hope it helps, sorry for being long-winded
In regards to gunpowder units, its actually better to put them in two ranks.
Rotating actually takes LONGER than reloading in this game. This is because the units organize themselves into PERFECT lines every time prior to firing, and they cant seem to figure out how to walk past each other.