The backstab spots are the red circles. There is one Backstab spot at each of the following Locations:
The Backstab spots are used by players during step 4 of Phase II. The rulebook explains how Backstabbing works but neglects to point out that the red circle is what is used to indicate that a player is Backstabbing.
For more information on Backstabbing, see pages 16 and 17 which cover the mechanics in detail.