This is the crown of the Holy Roman Empire, which historians agree was not really holy or Roman, plus the “empire” part at the time was mostly wishful thinking. But it’s pretty impressive all the same. It was made in about the year 960. The cross denotes the holy part, the arch from front to back recalls the helmets worn by those great soldiers the Romans, and the panels all around denote parts of the empire.
The crown was apparently good enough for Charlemagne, who had himself painted in it in about 1460.