Six onion-roof towers supporting heavens

[Jorge_Stolfi]

This brings us back to Timbuktu.  As can be seen on the map from 1830, there were no round domes. How could there be? Such structures cannot be built from clay.
The Catalan World Atlas map was drawn from a purely European perspective using information provided by the Moors.

Sure.  And not all churches/mosques outside Europe had onion domes.

But the point is that, in the mind of the cartographer and his readers, onion domes were a Muslim/Asian/Orthodox thing.  Including Russia (check the map). 

And that notion cannot have sprung in his mind from nowhere...

All the best, --stolfi