His citadel is at the end of time orbiting the last dying star as entropy rules outside. But he rules an empire across large swathes of space-time. He has generals to serve him in many eras in a complex web that only he fully comprehends. For instance diamonds mined in the 17th Century (Earth time) fund arms factories in the 45th Century, and their weapons are used against rebels in the 38th Century.
Now he considers his birth Century the 31st. A time of peace with no challenges or threats. But the man who would be Kang studied the theory of time machines, a subject which had been banned in the 29th Century following the Ravonna Paradox. He found his answers in the notebooks of the 21st Century Victor Von Doom.
He built a time machine and went back to Ancient Egypt seeking the secret of immortality. But they didn't actually have that secret so instead he armed an army with future weapons and became Pharaoh Rama-Tut. He compelled slaves to build pyramids in which he stored for future use more futuristic tech he had designed.
Then he headed home to the 31st Century but overshot and landed in the 41st. He donned a timesuit he'd also designed, and became Kang the Conqueror and conquered the Earth of 4086 using his knowledge of military tactics from all of history.
But now he needed money to fund further conquest. He learned of a tomb of an unknown ruler containing a treasure of gems and rare metals from all over the Galaxy. He broke in and discovered that the tomb was his own (seemingly after he became Rama-Tut II). Kang gained 2 things from that encounter, as well as whatever riches were there:- A mask which completed his outfit, and a determination to change his future so that he would never be forgotten.
And he went on to create his time-space empire.
Now he prepares to face his enemies. But his accumulated armoury disappears, wiped from time. As do his generals (or at least their transtemporal video-links). And finally his armour dissolves except for the mask. And his enemies break in - an assembly of Avengers from different times.