Eugene's World

Eugene's World was a pocket universe created as a prison for Eugene Tacitus, after he had murdered his own son.

Society
The world took the form of a medieval castle. The prime weapons were swords, but firearms were occasionally used. The people were ruled over by a King, who was proclaimed a God. The King would rule as a (mostly benevolent) despot, with all choices and will coming from the King. A scribe would follow the King and record all events into a Holy Journal.

In reality, the castle was purely a constructed world. Nothing existed outside the castle -- indeed, the people had no concept of "outside". Eugene Tacitus, the King's Scribe, was the only real person in the realm. All others were merely mental constructs. (BFA: The Holy Terror)

The Lesser Cycle
Traditions, rituals, and events followed a very specific loop in this world. Upon the death of the previous King, his elder son would be proclaimed the new King. Having proved the old King to be a false god (as evidenced by dying), any who still revered the old King were viewed as heretics. However, the people's loyalties and worship were easily switched to the new King.

The old Queen would be imprisoned, tortured, and killed. The new King's younger brother would lead a rebellion against the new King's rule. The rebellion, aided by the High Priest, would fail and the two conspirators would be executed. The new King and Queen would raise two sons in a loveless marriage, and the cycle would repeat upon the new King's eventual death. (BFA: The Holy Terror)

The Greater Cycle
After more cycles than of Royal Succession than Tacitus could remember, a different and larger cycle unfolded. The King's brother would raise a son of his own as an all-destroying God. That child would then kill every person in the castle, looking for his father, who in (conceptual) reality was Tacitus. On the final confrontation with Tacitus, the Scribe would kill the child, repeating his original crime. The world would collapse and be re-created to run the Royal Successions anew.

Into the end of one such cycle stepped the Sixth Doctor and Frobisher. The world was structurally similar to the extra-dimensional space created inside a TARDIS. It was this that drew the Doctor's TARDIS to it.

The new (and last) King abdicated his throne to Frobisher, who attempted to break the cycle and give Free Will back to the people. Unfortunately, the people were massacred before Frobisher could get them to understand the concept. The Doctor also attempted to get Tacitus to break the larger cycle by convincing Tacitus not to kill his child. However, Tacitus could not endure either continuation or escape, and had his son kill him instead, permanently breaking the cycles. (BFA: The Holy Terror)