Bulgaria trades city assault power for a genuinely different war economy — pillaging enemy territory funds your own cities directly, turning raiding into a core strategy rather than a side effect of conquest.
"Krum's Dynasty" grants Production to all your Cities equal to a percentage of the yield or HP gained whenever you pillage a Building or Improvement — but all Combat Units get decreased Combat Strength attacking Fortified Districts specifically, a real trade-off that pushes Bulgaria toward raiding the countryside rather than storming city walls directly.
The Bolyar is a unique Cavalry unit that ignores Movement penalties and enemy Combat Strength bonuses on Rough terrain, and gets +3 Combat Strength of its own fighting there. The Tarkhan is a unique Commander that lets units in its command radius pillage for reduced Movement cost, letting a raiding army sweep through enemy territory unusually fast.
The Hidden Fortress is Bulgaria's unique Improvement — it grants Production, Stealth to units on the tile, counts as a Fortification, and adds Combat Strength, but can only be built on Rough terrain and can't be adjacent to another Hidden Fortress. The associated Wonder, Rila Monastery, grants Culture, Relic slots, and a free Relic whenever any Wonder (including itself) is completed.