In the original script, both the excavation machine and a translating device which was intended to float beside the Gravis were originally envisioned as being made from the remains of the human colonists kidnapped by the Tractators. These ideas were deemed too gruesome, however, and were scaled back as much as possible.
Lesley Dunlop had not been offered or considered by John Nathan-Turner to be a regular companion. If so, the story would had ended with Norna deciding to join The Doctor, Tegan and Turlough aboard the TARDIS as the 4th member of the TARDIS crew.
This episode was watched by 5.6 million viewers on its original transmission.