Development of the ancient Roman army
The ancient Roman army developed probably from bands that lived of plundering raids, like the other peoples in the neighborhood. Possibly their leaders were the ancestors of those who would become the patricians. Round shields and simple armament if any. Also the famous gladius, the short sword, wasn't there yet. Because the early Romans didn't burry their dead with their weapons and armor, it's unknown how they really looked like, but it may have looked like this.
No brilliant tactics yet: Steal the cattle and get away with it. When you have to fight: everybody picks his man and tries and kills him.
Romans with Celtic helmets?
The well known Roman helmet wasn't Roman at all. It was a Celtic helmet, that was introduced to the Etruscans in the early fourth century. On their turn the Roman army adopted this helmet, it was certainly not the best type of helmet though. But it was cheap and easier to produce. That is a good thing when you need so many. For more helmets click here
So the Romans started the conquest of their neighbors with this helmet, a round shield and a spear. To protect their chest, the rich soldiers would wear chain mail, the less rich would wear a breastplate like the one at the warrior above. The famous segmented harnass (lorica segmentata) was first introduced in the imperial age. This worked fine until the ancient Roman army met the Samnites. This people had, like other peoples in South Italy, the habit of throwing javelins. The round shield , that worked fine in close man to man combat,didn't protect the body efficiently against thrown projectiles. The Samnites therefore had large, oblong shields, which gave them a better protection.
The Roman shields
The scutum not always looked like the picture here. It has been flat and oval, flat and formed like a trapezium etc. The ancient Roman army not only copied the scutum, which through various forms evaluated into the scutum as we know it, they also copied gradually a new tactic.
Instead of the inflexible phalanx, they introduced the maniple, which were blocks of about 160 legionnaires
The javelin evolved into the pilum. The long metal shaft behind the point was flexible and would bend when the pilum hit anything. This prevented the weapon being thrown back. Above that when the pilum got stuck in a shield it was very difficult to remove it. Well thrown pila often got stuck in a shield. This meant a great disadvantage for the owner of that shield, as it would hinder him in his movements.
The gladius, the famous Roman short sword wasn't a Roman invention either: it was Spanish or more precise: Iberian. The weapon was designed for thrusting and stabbing, which worked fine with the big and heavy scutum.