Romans had scorpios and trebuchets- artillary.
Your history is just a little off: the Romans had ballistas, not trebuchets - they had a few centuries to go before that beast showed up.
Siege engines were useful but not mobile enough to support infantry attacks - the primary mode of rapid attack was via cavalry, which both the Japanese forces and the Romans employed.
The problem we both have is anachronism: the Samurai and its warfighting tactics came into being about 600 years after Rome disappeared from world power.
We are doing the “what if the Confederates had AK-47s” silly stuff - so we’re both wrong!
And not to be overly picky, but “artillery” is spelled with an “e” in the middle; I was a career artillerist, and we contend that “Artillery lends dignity to what would otherwise be a vulgar brawl”.