The holy roman empire is a kind of alliance between factions in and around germany. There are around 21 factions in it and usually if you are at war with one the rest in turn declare war on you. Each 21 factions have their own units and can even declare war on each other.
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When you start the game the holy roman empire emperor is the holy roman empire faction that owns 2 provinces. I am going to do unit info for that faction later. Below are the other 20 factions which can declare war on each other etc but are in the holy roman empire. If you conquer land in the holy roman empire a message will pop up that the holy roman emperor will not accept your claim for the land.
FACTIONS IN THE HOLY ROMAN EMPIRE
Holy Roman Empire Faction (Faction, 2 Regions) - Website unit + start guide
Commune of Bologna
Republic of Pisa
Commune of Verona
Commune of Milan
Republic of Genoa
Canton of Schwyz
County of Savoy
Duchy of Bravia
Duchy of Austria - 2 Regions
Kingdom of Bohemia
Duchy of Brabant
Electroraite of Trier
Duchy of Lorraine
Upstals Boom League
County of Province
Margravate of Brandenburg
Duchy of Saxony - Where the Queen of the UK is from.
Dolphine of Viennois
The Holy Roman Empire Alliance keeps changing as the game goes on - Factions are added and leave.
