Here's a scheme of "line of sight" among the ruins that I know of in the area, including the Circular Fort. There's actually another hilltop fort west of Sears Kay that I've only recently learned about.
What's notable is that it's the relatively flat plain that leads to the lower Verde area (and all its ruins) which is being watched over. It confirms to me the thesis that they were on the lookout for attacks from the south. Especially with the fort I've recently learned about that's just to the west, these also keep an eye on the entrance to the Camp Creek route that could be taken north to the Agua Fria plateau.