Complex sentence: The Empress called upon her favourite architect, Charles Cameron, to design this "Byzantinesque" church, but the Scottish architect, though well versed in the Palladian idiom, had a vaque idea of what Byzantine architecture stood for.
Split into simpler sentences: The Empress called upon her favourite architect, Charles Cameron, to design this "Byzantinesque" church. However, the Scottish architect, though well versed in the Palladian idiom, had a vaque idea of what Byzantine architecture stood for.