Everlasting Imperial Diet of Regensburg

You aren't wrong to think of Vienna as the Capital of the Holy Roman Empire, but from 1663-1806, it's Diet was in Regensburg. I recommend the guided tour.

Regensburg City Hall and the Imperial Hall Building are adjacent.

According to my notes, in this room Electors got drunk and negotiated who would be Holy Roman Emperor.

The Imperial Diet was the legislative body of the Holy Roman Empire. The first business of the Everlasting Imperial Diet of 1663-1806, was to organise a defense of Vienna against attack from the Ottoman Empire. It ended in 1806 when Napoleon dissolved the Holy Roman Empire.

There were two kinds of Electorates or Principalities: Free, and Ecclesiastical. I believe that secular envoys would sit on one side, ecclesiastical on the other (but I could be wrong). Of course what was missing here was the Third Estate. In many cities you would find a Rathaus where a city council representing the Guilds and Mercheant class would compete for power with the local Prince-Elector or Prince-Bishop, but there was no representation for the Third Estate in the Diet.

By convention the Romans would call the main gate for their military camps Porta Praetoria.

Castra Regina was founded by Emperor Marcus Aurelius.