The way it was described to me by players I used to play with at another site was that
England fights Denmark
France fights Cordovia
Germany Fights Rus
Abbisids fights Byzantines
The first 3 do it for the Armies, The Abbs do it for promotions and expansion to the South shore of the Black Sea, The Byzantines receives promotions and hardens Constantinople and secures the Mediterranean(optional because of gold)
When the Christians are done with their wars they fight Constantinople but it is a tough fight because it is basically 100% of the Byz production defending it, that is why there is a narrow land bridge beside it, so all 3 have a space to hit it from.
This raises Byz points very close to victory and becomes a race of jealous Christians to take Jerusalem.
I was told it was nearly impossible for the Christians to win because the Abbs are to powerful and the Christians know only one of them can win. So it becomes a free for all on the final stretch and it is here that the abbs win it most of the time even if is a High level AI.
What I thought was going to happen was Germany was going to fight me, if he had I would have crumbled easily threw most of the game, but he stayed a true friendly neighbor and virtually gave me every tech. having been able to research probably 4 or 5 techs of my own.
I don't know how each civ was set up but I can tell you from beginning to end the Byz were always facing corruption, the bigger it grew the more corrupt it became. The buildings that negate corruption were not on the Byz tech tree and these techs were given or traded to me by Germany, if not it couldn't support enough units to defend critical cities, let alone of all them