Possible Clues:

• Realm first led by Augustus
• Bygone Mediterranean superpower
• Cassius Dio said it went 'from a kingdom of gold to one of rust and iron'
• Subject of a classic six-volume work by Edward Gibbon
• It fell historically
• It was established by Augustus
• Powerful international organisation that could make Premier moan
• What began with the reign of Augustus in twenty seven BC
• Edward Gibbon's subject