You didn't mention the velites. Polybius included a fairly informative description of the Roman military in his "Universal History". I have a Penguin Books book by the name of "The Rise of the Roman Empire". Polybius talks about how the legions are enrolled, why he thought the Romans were courageous soldiers, their types of awards, how they laid out their camps and some detail about discipline in the camps. The Roman Censor graded the citizenry by wealth. The velites were the lowest because they were 18-20 year olds, the 20 somethings hastati were next, the prime 30 type principes were next and the 40 something triarii were the highest graded infantry. A man was obligated by law to serve around 20 annual legionary enrollements before he reached 48-50 years old. I think according to Polybius, the republic legion of the early second century BC had a nominal strength of 4200 infantry, although this varied. There would be about 1200 velites, 1200 hastati, 1200 principes and 600 triarii. My understanding is the velites were light infantry carrying small shields and a bundle of 4 and a half foot long javalins. They ran toward the enemy line and threw each of their javalins and then raced back through openings in the Roman line. The other three classes were heavy infantry with the big shields, all the armor and the short swords. After the velites had lodged their javalins in the shields of the enemy, the heavy infantry lines would engage with the hastati who were in front and the principes who were second spelling each other. The triari who were behind them all had long thrusting spears and they would kneel with the butt end of the spear on the ground at their knee and the spear pointed forward while the fighting proceeded. There doesn't seem to be a lot of latitude here for tactics. But the testudo was probably an urban or asymmetrical warfare adaptation. After all soldiers in this configuration can't use their swords easily so they would not be worried about close quarters contact with effective infantry but rather about missiles from people out of reach of their swords. Polybius described the javalin or pilus as having a 4 foot long haft 6 inches into one end would be embedded a foot long iron rod with a barb on the outside end. This rod or point would be annealed or softened in order that after it was thrown into an enemy's shield the weight of the haft would bend the point so that it would not be thrown back at the Romans. I suspect the real purpose was to make it harder to remove it so that it would weigh down the shield during the battle. The Romans Polybius described prided themselves on being able to adapt to different situations in order to win.

I think that the Romans were also the best Combat Engineers in ancient history. And maybe not just ancient history. It was nothing for them to put up 40 miles of fortifications.

Reading Plutarch's life of Pyrrhus, King Pyrrhus saw his first Roman camp when he chose to do his own reconnaisance. Plutarch wrote that the sight of the Roman camp filled Pyrrhus with wonder and amazement. Polybius describes a republic era Roman consular army camp. But first, he describes how soldiers moved out ahead of the marching army to the prospective camp site and marked its layout with color coded flags. Judging by his description of the streets and the rows of tents for the praetor, the infantry and the cavalry, etc a typical consular army camp was laid out on a square 2,150 feet on a side or about 700 yards. The entire camp perimeter would be a little over 2,800 yards or 1.6 miles. The camp perimeter fortifications consisted of something like an eight foot ditch with the removed dirt forming an adjoining parapet of the same magnitude in height. The parapet was topped by a pallisade consisting of cut tree branches/trunks with sharpened points and I believe with woven sticks around them for bracing. The army carried these to the camp along with their picks and shovels. Imagine a consular army on the march in hostile territory camping at a different spot every night. Clearing the trees and brush and doing the digging and then putting up the tents. Then posting the watch and preparing the food and getting on with the routine.