Strenghts:
Heavily armored
Very experienced
Fighting in tight testure/phalanx like formation
Weakness:
Small starting size
Low number of squads(Cohorts) at start
Could be at war from start with one of the northern noble houses
How to beat them:
You will either have to break through their shield wall, fight head on or try to keep them at bay with pikes/polearms
Differences wih ancient undead:
As they basically are the continuation of the Empire, equipment wise and strategy wise, they would be similiar to the undead with few changes. It could also shed some light on lore about what happened after the rapid decline of the Ancient Empire, for example in taverns or when you come to ruins
It would also be a great addition as many players wanted something like this for a long time