With a piece of chalk on a solid surface, or perhaps a finger in dirt, a quickly drawn Circle is liable to be easily broken (even by accident), ending spells cast upon it.
But with enough preparation at the location you intend to use such Circle/Ring spells, you can make a much more enduring circle.
Originally I envisioned a solid stone floor with a circular groove of significant depth and width filled with solidified molten lead.
Now I am enamoured of the idea of simple, raised circular dais.
I think it depends on what you say breaks the circle.
If any obstruction, a twig, a rope, part of a foot, breaks the circle (and I personally think it should), the raised circular dias is much harder to break, as things like a rope or twig may fall off, the person breaking the raised dias circle with a foot needs to maintain balance. With an inlaid circle, those issue for the circle breaker don’t happen.
One could argue a raised dias would be easier to chip, thus breaking the circle.
My character just enchanted a stylus with a steel tip to be able to write/carve in stone/metal/wood and leather (a MuTe base 4 to make it highly unnatural/grant a property, additional level to affect different supports, +2 because the tip is made of alchemical steel, etc…). He is becoming a ward specialist and thought that such tool would become quite useful to have around.
I think there’s an unwritten assumption about Circle/Ring that because the combo is so temptingly abusive to game balance that it is supposed to be easily broken. Even if you get all tricksy clever and cast it on an inlaid circle of hardened steel that’s out of reach or somesuch.
It’s modeling stories where the magic circle is broken by the chalk line being scuffed or washed away, a leaf fallong on the circle, someone stepping on the circle, etc. The efforts a warder puts into making the circle unbreakable don’t mean that it won’t eventually be broken – it’ll just reduce the probability of it happening at any time; eventually something will happen and the circle will be broken. A spider will spin its web over the circle inlaid high into the wallls of your circular lab and suddenly demons can get in.
However, I’d like it if someone got clever about enduring Circle spells because it’s a good opportunity for Stories when whatever they’re relying on inevitably gets foiled.
Making a enduring circle is a challenge in itself, and if successful deserve to get reward. The reason the stylus I mentioned will be useful it is because my magi with use the Ring/Circle magic of the Columbae, requiring only to write symbols on the opening of the room/building to be warded.
I have had some long talks with my players with the intent of making the both useful, and not feeling like cheating.
Most of our decisions leaned towards the Form involved. A Creo Corpus healing circle will break if any corpus goes across it. A Circle Corpus ward will break if something Corpus steps across it. The other possiblilty is the medium the circle is on - a stone platform with a carved circle inlaid with lead will care when metal falls across the circle, but doesn’t care if a person or leaf flops onto it.
My original troupe agreed that ring/circle felt like too easy a shot at “permanent”, so we took the rule from Portal that the ring cannot be moved.