It's not the military tactics that allowed the Romans to succeed in the battles for their empire, but an economic and logistic system that allowed them to fund a standing army in the hundreds of thousands.

Meaning during particular campaigns the Romans were able to bring armies of between 30,000 to 100,000 strong which could remain in the field for quite a long time, in comparison to medieval armies that at their peak rarely exceeded 15,000 to 20,000 and were even more dependent upon the summer campaign season.