Roman Line won't hold...what am I doing wrong.

[Spuddicus]

- screen with velites and archers, ie put them before, not behind your hastati. Surely that is the historical deployment? Friendly fire is horrible in RTW. Now, the velites may not do much directly against a barbarian infantry charge but I believe in TW, charge bonuses only work against the unit charged. The barbs charge your velites; they have to redirect to hit the hastati as the velites pull back. On medium, I have found the barbs sometimes halt in confusion and even retire under terrible fire when the velites give way to serried rows of hastati.

That's the way I do it.
My formation for offense and defense is basically the same -
Heavy Infantry in the center - lines expanded in ranks of 5-6 deep (large unit size), with the General right behind them.
Way out on my flank, effectively detached from my main force is my calvary. If I can hide them behind a hill, even better.
In between my main Infantry force and my calvary are my 'spec ops' units (dogs, pigs, gladiators).
On my other flank I'll usually have one or two units of infantry (closer to my main force that my clavary flank).
Both flanks are deployed slightly forward of my center in a semi-flattened 'U'.
Velites up front and center, with archers right behind them.
On offense, move the whole lot forward slowly until your velites are in range; usually the enemy will react in some fashion and that's typically the que to pull your flanks in.
On defense, kick back and watch the bloodbath (again, your flanks are key here).

[TheDuck]

Ok, lets clear one thing up.

There is no 'proper historical formation' per se. I agree that when it was convenient the 3 line formation was respected. But most generals that were at least 'ok' decided on frontage based on what they were seeing on the battlefield. There was great concern on ancient battlefields of getting the 'flank turned'. So frontage was always of primary consideration. And there are frequent historical references to deploying troops so as to prevent just such an occurrence. Even with Roman articulated-manipular infantry, fighting in more than one direction was just bad.

It also seems that different generals had different opinions about where their more questionably mercenary auxiliaries should go, so that isn't even clear cut.

What was 'preferred' is not always what was 'used'. Its all about practical application.

So adjust your frontage so that the Gauls can't turn your flank, and use a reserve of one or two cohorts to plug the gaps. You can revert to 3 lines when your frontage allows that (just like most Roman generals did) and still respect history.

Given the very slight difference in Hastati and Principes in the game, I tend to run w/ 8 Hastati and 4 Spear Mercs then Triarii afterwards. I play just as you do, but adjust frontage accordingly (just like ancient generals), and find my guys do fine. The game's mechanics even supports identifying the 'veterans' (since they show up as 'chevroned' hastati infantry). So when deploying in 3 line formation, I put my more experienced guys to the rear as my battle reserve. This works really well because they provide just the right amount of shock at typically the pivotal moment in a battle (when all lines are fully engaged).

The truly great generals of ancient times showed one quality above others which made them successful. They adapted to what they saw.

[Spuddicus]

And there are historical examples of this 'flexibility'.
First one that comes to mind is the battle of Marathon.
Although it took place 200 years prior to the Punic Wars and involved the Greeks and Persians, it's a classic example of a general adapting to the situation.
First off, instead of the Hoplites advancing in slow tightly, packed Phalanx formation, Miltiades (I think) ordered his men to spread their formations and charge.
Seeing a Greek Hoplite army running towards them scared the bejesus out of the Persians, who quickly tried to reform a rear guard as the rest of their troops scattered back to their boats.
Miltiades then put his weaker troops in the center and fanned his flanks out in a very risky attempt to match the Persians. As the Persian engaged the center, he brought his flanks in and drove the Persians from the beach (and into a swap).
If you think about it, Marathon really was the first Cannae.

[Ulstan]

Firstly I highly recommend the 75% kill rate mod and the ~80% move rate mod. That helps a lot.

Secondly, once you get a few bars of experience on your men, or a good general, they will be much less likely to route.

Oh, and play on medium for sure. I'd rather play on medium and just build smaller armies than the computer than play on very hard and have unit balance all out of whack.

I usually deploy my hastati in a wide formation, 2 deep, to cover as much frontage as possible. It's not terribly historical perhaps, but though they will give ground under a charge from barbarians they won't break and run with a good general. I had one unit cut down to 3 men in a fight in a forest vs gauls and they still didn't run.

The barbarian noble cavalry will, however, go right through 2 ranks of men so you have to watch out for that.

At the flanks, I put a unit of hastati in deep column formation, slightly deeper than it is wide, but pretty much square. Cavalry charging into that inevitably get bogged down and killed.

I have maybe one unit of equites to chase routers, other than that I rely on the infantry and my velites + archers. Put the skirmishers in front of your men, and have them fire away till the last possible second then run behind. (You will need to turn off fire at will or your archers will happily just shoot your own men in the back).

Leave hastati on fire at will as the enemy charges. Javelin throws, especially once you get a few weapons upgrades and exp chevrons, can by themselves send a gaul unit fleeing temporarily.