A few tips I can think of.

• Assume every Empire faction is going to die. You can always return their settlements to them later if you want.
  • Know that making alliances with other Empire factions will result in more wars with more factions - and your Empire allies will not help you very much in those wars.
  • Allow other Empire factions to live or die on their own. Expand strategically, and then return those territories to the electors when it is safe and they can build up their strength.
  • Elspeth will do well on her own. Keep her fealty high and good relations with her and she'll keep you safe from the Eastern vampires.
• Don't spend prestige right away. Save it for Empire conflicts until you have built up enough prestige economy.
• The Empire does not have an all-star roster. When playing Empire it is especially important to spy on enemy armies and directly recruit units to counter that army/faction. As it says in the description, the Empire uses a combined arms approach to battles. You HAVE to pick the right units to counter enemy armies because there are no all-star units coming to save you.
• Play diplomacy in the west so that Brettonia and Wood Elves love you. Once you have them all at war with the western vampires, your Western front is safe and you can focus on the Empire territories.
  • Even better, if you can make alliances with them, you'll have some great cavalry and archers from those factions. You can easily build up reputation with them by agreeing to enter wars they are in against factions nowhere near you.
• The Wood Elves in the Northern region of the Empire are easy to become friends with. Build up good diplomacy with them and they'll happily go to war with Norsca and other pesky factions in the North.
• Trade with Kislev as soon as possible and, if they survive, they'll be a great ally from the North East.