It was during production of this story that Jacqueline Hill decided that she was leaving the series.
In a nice piece of internal continuity, William Russell starts gently banging his fists together as he leaves the TARDIS interior set and carries this through to the next scene, following a recording break, as he emerges from the police box onto the Xeros surface set; this gives the effect of a continuous piece of action and helps maintain the illusion that the TARDIS interior really is inside the police box shell.
Glyn Jones was later dismayed by Dennis Spooner's editing of his scripts, which excised much of the humourous content. Spooner felt that such material was inappropriate in what he envisioned as a high-concept science-fiction story.
The episode begins with a brief reprise of The Warlords (1965), which is currently the only surviving 16mm black & white film footage from that episode.
A better quality print of the episode was returned to the BBC in 1981.