When Patrick Troughton saw rats at the location, he threatened to quit on the spot if he saw any more. During World War II, he had rat-shooting duties and this upset him greatly.
The last Doctor Who serial to be filmed and transmitted in black and white.
It was David Maloney who suggested that the alien mesmerism should be manifested by the officers putting on spectacles. Terrance Dicks and Malcolm Hulke had originally conceived them as speaking in a cold "alien voice" when exercising the mind control.
Filmed on a rubbish tip in Brighton which had previously been used in Oh! What a Lovely War (1969). The Rats who lived there worked on both productions.