Ancient Roman Battle Organization

I copied these notes from various Wikipedia articles and images from the web.

Preparation for Battle 

"Once the legion (about 3,000-6,000 men) had deployed on an operation, they would generally march to their objective. There were exceptions when the armies were transported by the Roman navy but even then in most instances this was followed by a march of several days or weeks. The approach to the battlefield was made in several columns, enhancing maneuver. Typically a strong vanguard preceded the main body, and included scouts, cavalry and light troops. A tribune or other officer often accompanied the vanguard to survey the terrain for possible camp locations. Flank and recon elements were also deployed to provide the usual covering security. Behind the vanguard came the main body of heavy infantry. Each legion marched as a distinct formation and was accompanied by its own baggage train.

At the end of a day's march, the Romans would typically establish a strong field camp called a castra, complete with palisade and a deep ditch, providing a basis for supply storage, troop marshaling and defense. Streets were laid out, units designated to take specific places, and guards posted at carefully designed gates. Construction could take between 2 and 5 hours with part of the army laboring, while the rest stood guard, depending on the tactical situation. No other ancient army persisted over such a long period in systematic camp construction like the Romans, even if the army rested for only a single day. This concentration of conservative security in deployment was mirrored both in the measured tactics of engagement for the infantry and by the largely conservative operational strategies employed."

I think this is a castra for a large slowly advancing army.  Probably smaller sized units would build much smaller castras.

Tactical Engagement

"The soldiers were long-term service professionals whose interest lay in receiving a large pension and an allocation of land on retirement from the army, rather than in seeking glory on the battlefield as a warrior. The tactics of engagement largely reflected this, concentrating on maintaining formation order and protecting individual troops rather than pushing aggressively to destroy the maximum number of enemy troops in a wild charge.

 The front ranks usually cast their pila, and the following ranks hurled theirs over the heads of the front-line fighters.

If a cast pilum did not cause direct death or injury, they were so designed that the hard iron triangular points would stick into enemy shields, bending on their soft metal shafts, weighing down the shields and making them unusable.

After the pila were cast, the soldiers then drew their swords and engaged the enemy. However, rather than charging as might be assumed, great emphasis was placed on the protection gained from sheltering behind the scutum (sheild) and remaining unexposed, stabbing out from behind the protection of the shield whenever an exposed enemy presented himself. Fresh troops were fed in from the rear, through the "checkboard" arrangement, to relieve the injured and exhausted further ahead."

Testudo (tortoise) Formation

"In the testudo formation, the men would align their shields to form a packed formation covered with shields on the front and top. The first row of men, possibly excluding the men on the flanks, would hold their shields from about the height of their shins to their eyes, so as to cover the formation's front. The shields would be held in such a way that they presented a shield wall to all sides. The men in the back ranks would place their shields over their heads to protect the formation from above, balancing the shields on their helmets, overlapping them."

Preparatory Bombardment

"Many Roman battles, especially during the late empire, were fought with the preparatory bombardment from ballistas and onagers.

These war machines, a form of ancient artillery, launched arrows and large stones towards the enemy, proving most effective against close-order formations and structures."

Drawing the bowstring back with the winches twisted the already taut springs, storing the energy to fire the projectiles. The bronze or iron caps, which secured the torsion-bundles were adjustable by means of pins and peripheral holes, which allowed the weapon to be tuned for symmetrical power and for changing weather conditions.

The ballista was a highly accurate weapon (there are many accounts of single soldiers being picked off by ballista operators), but some design aspects meant it could compromise its accuracy for range.
The maximum range was over 500 yards (460 m), but effective combat range for many targets was far shorter.

The Romans continued the development of the ballista, and it became a highly prized and valued weapon in the army of the Roman Empire."
There were several sizes of ballista.