Starting in 867 as a Duke or higher with your capital in Iberia or Aquitaine while not holding the Burgundy county of Lyon, select a religion with the Pope as the head, go down one of the Learning lifestyles to pick up Scholar or Theologian ASAP, wait until the beginning of the following year prior to 880, and a mad monk will appear. Listen to his every insane whisper, give into his every demand, tick off the Pope at every turn. After about 5 years of event chain, assuming you go all-in on radical fundamentalist doctrine, you'll get the most broken version of Adoptionism as your religion. Enjoy painting the map with that, and debating your vassals to literal death.
The Struggle mechanics are fun, I do like the ease of marrying outside your religion family for alliances, and mercenary options, all depending if you're on the stage of conflict. The only downside is you have to be on the correct phase to get certain endings, with Dominance being the only one always active (even Interloper requires a specific one, and you're not even technically involved). This isn't made for quick conquest, though the system can be used to finish the Reconquista MUCH sooner than the 700 years it took IRL, or have the Muslims win, or revive a dead Pagan faith to claim it all, or Jarl Hastings the thing via Norse hybrid.
izigame.me
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