The Imperial Palace in Constantinopel.

The Emperor of the Terran Empire is almighty, and rules for as long as he isn't expulsed in an uprising, which is almost impossible due to Indoctrination and the heavy nanomodifications that literally stop any thoughts of dissent. Everything the Emperorm says is true, even if it contradicts something he himself has said before. Once again, the implants and Indoctrination explain why everyone takes this as true.
Emperors, when not elected, conquer their throne with civilian support or by military conquest. The position of Emperor isn't hereditary, but he can appoint his successor.

Because the Emperor isn't alleseeing and can't micromanage everything, there is an extensive government that makes decisions based on the Emperor's decrees.

The Emperor is aided by a hand-picked Council (usually made op from his friends and relatives and the occasional politician, military commander of director) that oversees parts of the government, the Empire's corporations and military forces. Councillors advise the Emperor on important decisions, but have no real influence on his decisions.
The Councilors do have the power to use their part of the Empire's power as they see fit, as long as it doesn't go out of the bounds the Emperor has set.
Of course, the Emperor has full information on the Council's dealings and can overturn all their decisions.

The Emperor Tiberius himself: Ruling for centuries and not a single dethroning attempt. How?

Emperor Tiberius IV (Mike Redfield), the Lord Emperor of the Terrans has several traits and aspects that cause his people to think of him as a living god, preventing any attempts to take the Terran throne.

  • The Emperor is the most powerful Terran alive. Unlike the normal Terran, the Emperor can manipulate reality with his thoughts. Though not very powerful compared to fantasy reality warpers, because his abilities are natural and biological in essence. His mental powers are mostly passive: everyone who hears his words takes them for true, no matter how ridiculous; this has a great effect because the Emperor, as a public figure, makes all his decrees and legislations on live television. The Emperor's presence causes all those around him to feel immense amounts of loyalty and trust towards him, as well as giving people hope and hapiness; this extends to those who hear him speak and are not physically around him. The same things cause dread, fear and sorrow in enemies. His more active psycic powers include (mass) mind reading, (mass) mind influencing, (mass) mind domination (forcing people to believe whatever he wants them to, make then do things, or drive them insane) and telekenisis, and some offensive powers as well: (Names taken from Star Wars and Mass Effect for familiarization; effects are roughly the same only on a more powerful level, though caused by natural reasons and not SPEHSS MAGICKS.) Force Throw, Force Choke, Force Lightning, Force Crush, Biotic Shockwave, Biotic Reave, and Biotic Slam. (He can also crush your bones with his thoughts, set you on fire, make your body explode or make it melt.) Though such powers are not exclusive to the Emperor, his are the strongest and largest. (Unnamed mental techniques allow the target to deflect, rebound, depower or disseminate these psychic powers.)
  • The Emperor can form a new body out of sheer willpower, and can turn to liquid or vapor whenever he chooses to. No-one else can do this...
  • The Emperor is the only person whose mind is strong, fast and large enough to micromanage the entirety of the Terran Dominion without going insane. He knows how to maintain unity without causing a massive amount of red tape to bog down the Dominion's operation.
  • The Emperor is very a charismatic, kind, open and public person who knows how to get things done, genuinely cares about every individual Terran and has no delusions of grandeur or superiority.
  • The Terrans are a loyal people who know their limits; if the ruler does a good job they don't oppose him. And the Emperor has done more than a good job: he propelled the Empire from an unstable local power to an interuniversal superpower.
  • The Emperor is the smartest, most intelligent Terran in history, and is personally responsible for the Dominion's technological edge.
  • The Emperor actually listens to his advisors and citizens.
  • Under Tiberius IV, the Terrans have attained a previously unknown quality of life, which he continues to defend determinately.
  • Bread and Circuses
  • On the darker side, BPA and the Inquisition ensure that no-one disobeys the Emperor.
  • The Emperor and a handful of his most trusted compatriots can seize functional control over the Defense Grids at any time, meaning that any rebellion will fall apart *cough* as soon as it begins. The people are told that this is a security measure. And it is, in multiple ways. While some of the more savvy persons understand that it also protects the Emperor from uprisings, the masses only see it as protection from alien invasion. After all, the Emperor protects...
  • Mega-companies have no political weight and aren't allowed to inferfere in citizens' personal lives.
  • Several governmental branches and organizations wield a great amount of power; some fields overlap between branches forcing them to work together whilst also leading to inter-governmental rivalry. This way no coalition can be formed to take down the Emperor, as it would quickly disintegrate (or literally disintegrate).
  • Of course, Nano-brain-manipulation and Indoctrination also keep him in power, as consciously dissenting is made impossible. (Notice: a speech which could be interpreted in multiple ways caused the Terran Civil War... that's the conflict that caused over 150 million deaths.)