The tour takes you by Old Sarum, you can get off on the return trip and explore here, it is a huge earthwork raised in about 500BC by Iron Age settlers and later occupied by Romans, Saxons and Normans. They built a castle and a royal palace, and by the mid-12th century it was a busy town with a fine new cathedral.
Lack of water and squabbles between church and military lead to the building of a new settlement by the river, now known as Salisbury. Old Sarum was abandoned and fell into ruin. If you want to take a look at the site you can get off the tour and rejoin later on - your ticket lasts all day.
You can find out more at the English Heritage Old Sarum page.
Please bear in mind the closing time of Old Sarum, before catching the later afternoon/early evening Tour buses.