Hi Lucy, I enjoyed this. I've read a few books about this period but am certainly no expert. I think you captured the atmosphere among soldiers very well though, what they were talking about and the way they were talking sounded very authentic.
As for Marcus, yes, he comes across as enigmatic, clearly respected, and I do want to get to know him, so well done!
I agree with Lucy about the length, I think for what you are saying in this scene 3,400 words is a lot, and I think there are areas you can edit it without losing any of the mood. A few small points from the first page, battlefields wouldn't be in the past. Only the battles themselves, and a fatal wound can't strike, maybe 'be inflicted', or 'where would the fatal blow strike'? These are just minor points though.
Overall I really enjoyed it (is it true the Romans were attacked as soon as they landed?)