Planet of the Boundary

The planet of the Boundary was the inhabitable but uninhabited world where the Boundary stood.

History
Seeking to pass through the boundary, human refugees from the Cyber-Wars once founded a settlement near the Boundary, under the leadership of Ko Sharmus, erstwhile member of the Alliance.

The Thirteenth Doctor, Ryan Sinclair, and Ethan arrived on the planet in a stolen Cyber-shuttle. Ko Sharmus showed them the Boundary, which appeared as a rippling sheet of energy. It unexpectedly cleared, and the group saw the capital city of Gallifrey on the other side. The Master leaped through the Boundary and informed the Doctor that "everything [was] about to change forever". (TV: Ascension of the Cybermen) By threatening the lives of her friends, the Master was able to force the Doctor to join him in traveling to Gallifrey while the humans remained behind.

As the three attempted to decide on a course of action, a Cybercarrier arrived with its army of hundreds of thousands of Cyber-Warrior. Anticipating an attack, Ko Sharmus led Ryan and Ethan to his weapons store before, as he expected, Ashad dispatched execution units to hunt down the humans on the orders of the Master. Using automated turrets and a bomb thrown by Ryan, the group managed to take out most of the Cybermen before being forced to flee inside by the advancing forces. Splitting up, the three managed to take out a few Cybermen before Ethan was captured and his life threatened to force Ko Sharmus and Ryan to surrender. The three were saved at the last moment by Yaz, Graham, Ravio and Yedlarmi who had escaped from the Cybercarrier by disguising themselves in Cyber-suits. With the Doctor, Ashad and the remaining Cybermen on Gallifrey, the group decided to cross the Boundary to help her. (TV: The Timeless Children)