Which is a really important point!
Beautiful to behold but often a nuissance at their best! My jury is still out on the whole ward issue. Can't entirely live with them, certainly cant live without them.
Another somewhat undecided issue in the rules what it takes to "break" the circle? Physically destroying it, just someone passing over it, or the enchanted thing(s) or being passing over it - leaving it?
On a sidenote to this whole Ring/Circle business it is really an interesting way to make seudo-permanent effects. We had a NPC-magus that had installed a broad band of bronze in the wall of his circular tower chamber. By the use of this he had made several effects that thus remained even years after his death - and at the time of our protagonists intrusion into the former magus' sanctum - including two bronze guardians made by human skeletons incased in bronze wielding wielding Greek weaponry. Well.