Honestly, if a Byz player chooses Crusade, they are nigh impossible to defeat. I've never seen an actually OP civ before, but I think this is it. However, in playing with a friend, I found on successful counter - play a very offensive religious game against them. Keeping them on the defensive with their religion (eg converting their cities preventing them from converting yours) is the only way to prevent their additive bonus cascade. When I say early, I mean either get a religion before them or ASAP once you know they are in the game. Having Byzantium in the game gives you no choice but to focus more on a religious game, because if you don't, they will destroy you. (This is particularly true with another human Byzantium - I haven't played with an AI one.) Anyone else have successful counter strategies?