In Spanish the letter 'x' is not pronounced /ks/; at that time it represented the initial consonant of 'sherry' (wine from Xerez). That is how the Spaniards used it in transcribing Nahuatl (the Aztecs' language).
The cameras used to film this episode were incapable of zooming - instead they had to be physically moved closer to their subject. This is apparent when the camera "zooms in" on Tlotoxl during the sacrifice scene and the camera lurches violently after hitting a piece of the scenery.
William Hartnell oversteps his mark during the first scene in the temple's main chamber after Barbara is deemed to be the spirit of Yetaxa. He delivers his subsequent dialogue with almost all of his face obscured by Barbara's headdress.
Susan asks Barbara if Hernán Cortés landed in Mexico in 1520 - Barbara, supposedly a specialist on the subject, confirms this. Cortés actually landed in 1519.