The Iron Wall

This formation works only on open field, so if you are forced on the defensive, choose to sail out of your city so you have all the space you need. If they do not attack, lure them to your lines with skirmishers, or any other fast unit.

Here is the goal, kill most of the forces, before they reach your fronline.

This way, not only your troops will have a easy time winning, but you can also take control when needed.

Here is the formation you need to take:

At the back, your general, but you can also place him at the front. Aslong as he is not on a horse.

Then, your Ballistas: Take the explosive shots, they are very important, they will make some troops even flee! But don't forget to make them stop firing if they get to close to you formation!

After that, Archers: You really need archers, they will kill the most troops.

Optional:
Skirmishers: You don't need them, but they do help, so if you have space and money left, then get them!

Horsemen: Also optional, it is quite nice to kill the fleeing enemy, so get a horsemen or 2

At last! The Melee infantry: They wil kill and cut down any men who reaches the front line.

This is how it is how it looks:

Here is how it looks after a battle

(My soldiers didn't flee, they are are killing the fleeing enemy. So I could make this nice screenshot.)

Troops in my legions: from back to front

-4x Attilery

-4x Archers/bowmen

-2x Velites (Skirmishers)

-8x Melee infantry (Hastati, Cohorts etc.)

-2X Horsemen.

And ofcourse one agent, wich doesn't really matter at the battlefield, but is always very good to have.

Don't worry! I know that this is bound to happen.
Luckily, Rome has a wide verity of melee infantry. Wich ones you need to pick, is up to you
and who you are happening to fight against. (If you happen to fight against horses a lot, then a logic option would be to recruit Triarii, or other spearmen.)

If you hold your ground well, than you can see at the end of the battle a nice line of dead enemies
in front of your living ones. :3

If they manage to almost break trough your lines, then try to support that line with your other soldiers. But, don't get flanked, if that happends then you WILL lose a lot of men!
This a one-way formation, a very powerfull one, but easily flankable, if caution is not taken.

Also, sorry for my bad English

Thanks for reading!