The PCs in my game are currently in a Regio at least several miles across, where they have encountered a surviving Roman outpost, a copy of Cologne. They have made a deal to solve some problems there and in return, want some of the craftsman the Romans have.
Now, I know that the Romans had much better masonry, engineering, road building, various types of smithing etc. But this is a single town of perhaps a 1000 people, that has been separate from the Empire for 800 years. Assuming reasonable access to materials, what levels of crafts would a single town have maintained, or even gotten better at?
I'm not running a serious nitty gritty super detailed historical game, and none of my players are history buffs. So being super accurate is not a huge concern, just roughly accurate is fine.
I'm also expecting my players to likely try and leverage these craftsmen into providing superior services and mucking around with the medieval paradigm a bit, so any thoughts on introducing roman roads and cement into the 1200s via the city of Cologne in Germany would be helpful too.
TMK around 100 A. C. Romans used very good hydraulic cement for acqueducts in Cologne, whose main ingredient was a local stone, the Trass from the Eifel ( en.wikipedia.org/wiki/Trass ). It is quite unlikely that Regio dwellers still have access to or knowledge of this stone, but this would be your decision.
Some new acqueducts were built in the middle ages, like the imposing Ponte delle Torri near Spoleto or the important one supplying the Fontana Maggiore in Perugia. So a few savvy engineers educated by the Order of Hermes might find employment in actual medieval projects, if that is your idea - and anyway recovering knowledge of hydraulic cement doesn't appear world shaking and paradigm breaking to me, given the precarious state of technical communication in the middle ages.
In the Europe of Ars Magica your engineers would then compete with experimental philosophers from A&A p.67ff, some of whom can make Cement (A&A p.75) with similar properties in a very different way.
EDIT: Added the wiki link, as not everybody knows Trass.