The dome in all its splendour. Of course when a church it would have been covered by at least frescoes but (because it was founded by emperors) more probably mosaics of the finest quality, just like most of the other surfaces. Walls would be covered with the finest marble. The fine "Strolling through Istanbul" makes a comparison with S. Vitale, Baptistry and S. Apollinare in Classe in Ravenna, which are of the same period. I have visited each of them several times, so I could fill in the picture a bit. Wow.