The seat of any government is certainly its capital city.

The city of Rome was the dragon's seat.

But in A.D.330, Constantine transferred the seat of empire from Rome to Constantinople; and Rome was given up to what?
To decay, desolation, and ruin?
No; but to become far more celebrated than it had ever before been, not as the seat of pagan emperors, but as the city of St.Peter's successors, the seat of a spiritual hierarchy which was not only to become more powerful than any secular prince, but through the magic of its fatal sorcery was to exercise dominion over the kings of the earth.

Thus was Rome given to the papacy; and the decree of Justinian, issued in 533, and carried into effect in 538, constituting the pope the head of all the churches and the corrector of heretics, was the investing of the papacy with that power and authority which the prophet foresaw.
It is very evident, therefore, that this leopard beast is a symbol of the papacy.

But there are other considerations which prove this.