User blog:MrsWillburn/Guard Post Locations?

I'm having trouble figuring out what constitutes a "guard post." I'm trying for the Courtesan Guild Challenge where Ezio needs to kill 20 guards at guard posts without being detected. I'm just wondering what these specific "guard posts" are, seeing how I've killed a whole ton of guards at posts (with a crossbow, undetected), but it still says 0/20. Any help would be much appreciated!