Ah, I remember now why I gave up building the scenario few years ago... May there be better luck this time!
Building scenario with text editor is just

It just crashes in the exe and visual studio call stack doesn't even show any DLL calls... hard to locate errors.
But I guess I can work around it with python and world builder.
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Anyway been thinking the Crusades scenario. I guess there should be some reason or prize to capture Holy land (Jerusalem) if you play catholic nations? And
how does AI react to war between distant nations? For example, would the Frankish empire send units to Middle East if it has war with turks or arabs?
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I found nice map that has good list of nations:
France (Franks)
England (Anglo-Saxons)
Normandy (Normans)
Holy Roman Empire (Franks?)
Spanish Kingdoms (Visigoths)
Russians (Kievan Rus)
Hungary (Magyars)
Scandinavia (Scandinavia)
Byzantine Empire (Byzantines)
Almoravids (Berbers?)
Caliphate of Cairo (Arabs)
Seljuk Turks (Turks)
States of Church (Papacy)
Poland (Maybe needed new leaderhead)
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Croatia,Serbia,Ireland,Scotland,Balts,Finns,Khazars etc
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And since the time line of scenario would be around 1000-1500 AD there could be somekind of Mongol invasion event.
Not sure which civs should be barbarians or should there be any.
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Which gave me idea that
maybe Byzantines could be a some sort of advanced natives? You could learn new technologies from them and learn skills that you can't learn in normal barbarian villages. And possibly you could send units to
Imperial guard and gain some bonus (gold and promotions etc).
Another idea for Byzantines is that they would work like the Papacy for the non-catholic nations. For example arabs would win the game if they capture Constantinople.
Maybe something like this could be considered to the main game too (in the future)? Down side of course is that player couldn't play Byzantines.