Besides what /u/HatMaster12 says, I'd add that other than the discipline and order (and probably the costumes), there's really very little in the opening scene of Rome that's realistic or accurate to the events being shown. The series depicts, in theory, the final attempt by Vercingetorix's troops to break out at Alesia. Instead, it shows a murky battle being fought in the backwoods of Gaul somewhere against a force more reminiscent of Caesar's descriptions of the Germans or of Tacitus' Caledonians than it does any Gauls that Caesar describes. Alesia was not an open battle and it did not take place in some random forest. Alesia was a siege, really a double siege, with Caesar investing the city and building impressive lines of fortification around the city, as well as a protective rampart to prevent the enemy's relieving army from attacking his rear. This is what Alesia looks like, the city was a hill-fort surrounded by a fertile plain, and even if the trees you see in that picture surrounding the fort existed in antiquity Caesar's lines were much further out, well past them. They weren't fighting in some murky forest and they weren't fighting in the open. The Gauls made several attempts on Caesar's lines over the course of some days. Their first assault failed when Caesar's German cavalry massed their horse and charged the relief army, wiping out the enemy's skirmishers and cavalry in the process. The second assault was an attempted night assault coordinated between both Vercingetorix's army and Commius'--this also failed. The final assault, which is what HBO is trying to portray, was a second daylight assault on Caesar's earthworks from both sides on a weakpoint that Commius had identified in the fortifications. That there were fortifications is very important, they saved Caesar's ass and he mentions them repeatedly. The Gauls brought with them grappling hooks and filled in Caesar's trenches during these assaults, and Caesar's troops were fighting surrounded against forces attacking them from the plain and coming down the hill. We know, for example, that a crucial part of the battle occurred when Vercingetorix failed to link up with Commius and Caesar managed to throw in his reserves--during this action the fortifications were of prime importance, separating the enemy's forces and allowing Labienus to withdraw the scattered cohorts that had borne the brunt of the assault within several of the forts, from which they were able to keep the two armies from linking up long enough for Caesar to commit himself decisively. Equally absurd is the depiction of the Gauls--they have no formation, they fight with basically no armor, and they go jumping around like undisciplined ninjas on coke. Caesar repeatedly emphasizes the discipline of the Gauls. The Helvetii are described as forming themselves into a phalanx, and their shields overlapped each other so closely that when Caesar's men threw their pila the spears penetrated multiple shields at once, pinning the Helvetii to each other. Even the Nervii, whom Caesar considered to be the most warlike of the Gauls, set up a carefully-constructed ambush and encircled his position, attacking with ferocity but had enough discipline to advance together and to wait until Caesar's troops were busy building camp--Caesar specifically says that the enemy had agreed to wait until they could see his baggage train, at the end of the column, so that they could all attack at once. Vercingetorix's army, and Commius', were both highly-disciplined and should not be thought of as dirty, bearded, poorly-armored swingers of long-ass swords jumping into the middle of Roman formations for no apparent reason.

I've said it before but I might as well say it again. I thoroughly enjoyed Rome, it was easily the most realistic sword-and-sandals flick I've ever seen. But in the end it's very much a sword-and-sandals flick, not a realistic depiction of antiquity

No, and no.

/u/Celebreth has written more extensively on this scene (and the general accuracy of Rome) than I have, so I’m pinging him in the hope he will chime in.

It must first be noted that our understanding of the mechanics of Roman battle in any period are murky at best. No ancient source directly describes the “face” of Roman battle. Neither the literary histories of writers such as Caesar or Ammianus, or the various tactical manuals that survive from the Principate and Late Antiquity describe how battle was (in theory) conducted, especially from the point of view of the “average” soldier. Battlefield maneuver and the experience of infantry combat are thus highly debated topics.

The Romans did have a system of cycling reserves, yet this appears to have been done on a unit, rather than individual, basis. As the cohort had become the principal organizational unit by Caesar’s day, it is likely that whole cohorts were rotated in and out of the front line, with the exact mechanics of this maneuver unknown. There is no evidence to suggest cycling of individual soldiers within a file, as the show suggests. Indeed, Roman formations of this time were not even organized by file, but rather by rank. It is not until Late Antiquity that units were constructed by file, with each contubernium (which in Caesar’s time had no tactical significance) forming a file in the line of battle. The maneuvering of reserve cohorts would definitely not have been done “under fire”, but rather during a lull in the fighting. While I certainly appreciate Rome not depicting combat as a disorganized melee, it’s interpretation is still highly inaccurate.

I have yet to see any depiction of Roman battle in film that is anyway accurate to current scholarly interpretations. Rome comes the closest, but it’s still inaccurate. Hollywood is terrible at depicting pre-modern combat in general, and I honestly can’t think of any film that accurately represents our understanding of Roman battle, or of Roman life.