Hi Lucy, I am a big fan of this period, reading authors such as Douglas Jackson and Conn Iggulden. I would also recommend visiting the Kings & Generals youtube channel as they cover all of Caesar's campaigns in the upcoming civil war your books are based in (they also cover several battles you mention in your chapter, Alesia, Gergovia, the Britain landings).
I immediately liked the style of you're writing, it rang the same chord as the aforementioned authors, particularly the atmosphere of the soldiers as Quintus introduces Marcus through his story. However, I think you could make some to tweaks to push it even further.
- Soldiers too polite: Legionaries were hard bastards who only respected strength. They pillaged, burned, and killed everything else. Make us feel like these are intimidating men.
- Too many battles: Nervian Ambush (slightly vague), Alesia, Gergoiva & the Britain landings. While I know these different battles as I have an interest in the period, someone who is not may feel overwhelmed by these names. A suggestion could be to mention fewer battles and focus on those few, such as replacing the ambush that Quintus & Camillus talk of at the start with Gergovia, as it was here that your Main protagonist's problems start (the death of the Aquilifer, Lucius?) This, I feel, would be a nice way to foreshadow the problem facing Quintus- Marcus' apathy.
-Speech tags: You're allowed to say said! Something I noticed was that in between your dialogue you have a lot of actions & you use many attributives (proclaiming, piping up, interrupted, continued, commented- are a few I noted). Cutting some of the action & replacing these tags can go a long way to creating shorter, sharper engagements.
Overall though I enjoyed reading it! You captured my attention with the end hook, and i would eagerly read on to find out more about Marcus. Good luck, I hope you do well with this, you certainly have the material!