A. The monastery skins are said to be for “Byzantines, Ethiopians, and shamanistic civilizations”.
The Byzantine monastery should also be used for other non-Catholic civs using the Mediterranean set: Romans (pre-schism) and Bulgarians (Orthodox).
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B. Africa.
All of the new monk models had been created before or during the development of AoE2DE.
From the revealed 7 additions - 6 for the standard monk and 1 Pagan Priest, there is an 8th model, seen in an official slide, that’s missing from this update.
That is the sub-Saharan model, carrying a cross staff in the image above.
In fact, you can still find a remaining portrait icon of this monk in AoE2DE, holding a different staff and a whisk (a ritual tool in Ethiopian church).
While the original intention of this model (a model vaguely based on Ethiopian Church, to be used for both the Islamic Malians and the Christian Ethiopians) is bad, it remains a problem that the existing models will fit the two sub-Saharan civs poorly.
EDIT:
On closer look, the staff in both images might be the same - a cross staff with the top removed, in an attempt to make the monk not Christian.
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C. There are more complications with real history, such as civs that changed religions through AoE2 timeline, and further denominations.
The Bengalis are the perfect example:
The Bengal region began as a Buddhism-majority, became Hindu with the decline of Buddhism, then converted to Islam.
The in-game version is entirely based on the early Buddhist Bengali (campaign, unique tech, archaic unique unit), but that moves us to the other problem:
Different branches of Buddhist monks do not dress the same way.
the current model is based on Buddhist monks in Sinitic cultures (cultures that received Mahayana Buddhism through Chinese culture). Monks do not look the same in Theravada Southeast Asia, nor should they look the same in Pala dynasty Bengal.
We can ask more similar questions: should the Romans use the tonsured classical monk, or the very late Orthodox monk? And so on.