Ashworm dragoons sound good, presuming that we had firearms in this campaign. Still, a good cavalry.
Yes, this is in our campaign.
I'm the DM. I'm starting today, but I doubt they'll see the army for a couple of weeks.
Overview? There are bad guys trying to tear down the existing empires so they can build new ones and take over the world. Optionally, they can try to seize control of lesser power blocks and try to forge their new empires from them. During Rome's height, the Persian Empire was pretty much an also-ran. Rome dominated the region.
Now Rome is weak, and someone is manipulating things to pit a resurgent Persia against a declining Rome. Maybe it's to let them weaken each other, or maybe they've actually got some control in Persia. In either case, it's a problem.
PCs will be given some advance warning so they can high-tail it to Greece. The Governor in Thebes is a thoroughly indolent and corrupt failure, and his plan to deal with the advancing enemy is to withdraw all the "important people" (meaning the rich) to their various country estates, taking what military the city has with him as an escort. They'll empty the city's grainaries as they go, and essentially abandon the people to their fate.
That's the situation the PCs find developing when they arrive. What they do from there is up to them.
That will also decide whether they ever even see this army I'm working on. As high level PCs they can rip into a an army of low levels and do a lot of damage. But the army also has higher level types involved, a caster corps, monsters etc. The PCs would need to raise an army of their own, somehow.
If they decide to try and meet force with force, there are a very few places it can work. Thermopylae is one of them. There's a narrow isthmus connecting the Peloponesian region, where the local nobles plan to retreat to, to the major land mass of Greece. That's another pinch point.
Both have significant weaknesses, defensively speaking.
What they'll do, I really don't know. But I have to be prepared for as many possibilities as I can.