This is Phil Barker's significant reworking of the De Bellis Multitudinis: Wargames Rules for Ancient and Medieval Battles – 3000 BC to 1500 AD system.
Basic scale, size, and system remain in place, but significant differences in troop and army capabilities make it a completely new game that plays very differently.
Related games from the same family:
De Bellis Antiquitatis: Quick Play Wargame Rules with Army Lists for Ancient and Medieval Battles
De Bellis Multitudinis: Wargames Rules for Ancient and Medieval Battles – 3000 BC to 1500 AD
De Bellis Renationis
Hordes of the Things