The Empire was once a powerful, successful, and extensive nation, and it endured for centuries without facing significant threats to its influence. The Imperial armies were feared far and wide. Although ruthless, the Empire also spread knowledge and technology wherever it went.
The glory days of the Empire are past, however. Today, it is no longer a highly centralized state. Instead, it is divided into dozens of individual territories governed by kings, dukes, counts, bishops, abbots or other rulers, collectively known as princes. Some territories are very large (like small kingdoms) and others are only a single town. These rulers are considered subjects of the Emperor.
There are also some areas ruled directly by the Emperor, though these are generally limited to areas around Sancta Civitas (the Holy City). Concerned with barbarian invasions and power plays from ambitious princes, the Emperor no longer has the power to simply issue decrees and govern autonomously over the Empire. The Empire is, in fact, fighting for its life against the invasion of Barbarian hordes from the east. Due to this, some western princes are trying to carve out their own kingdoms, conquering nearby territory and resisting Imperial rule. Menoa, a coastal kingdom to the west of Bridgeport, was the first to successfully break away from the Empire about half a century ago, but others are following.
The Emperor is considered to be the spiritual subject of the Patriarch of the Church. In reality, the Patriarch has far more political influence than the Emperor, as the Builder religion remains strong throughout the empire.
The language of the Empire was Latin for many centuries. As the influence of the Empire waned over the last few centuries, local languages began becoming more common, and now Latin is used outside the center of the Empire only in ceremonial and religious situations.
The "Empire" of the TDM setting is based on the Roman Empire of history. Imagine a Roman Empire that was not overrun by barbarians after a few centuries, but instead endured for a millennia or more. The barbarian invasions that destroyed our own Roman Empire have just begun in the TDM setting. Use the historical Holy Roman Empire of the 15-16th Century for inspiration and influence about how the Empire works.