The thing to remember about circles is that Wizards can cheat in several ways. For a non-magic user, say Butters, to empower a circle the options are pretty limited. He needs an actual physical circle to contain the energy, and then he dumps a blob of said energy into it with a drop of blood. It's a little unclear whether this is a one-time investment or setting up a link, but either way. Different materials conduct that energy differently (Cold Iron imparts the anti-fae properties on circles and many other spells), and to varying degrees of efficiency (blood conducts more energy, perfect circles are better than rough ones, etc.) Some circles require anchor points (see Harry's talk about the three-layered circle in FM), others dont.
But Wizards cheat. They can impart will and energy into the circle directly, and so they can do nearly anything a physical circle could do if he imagines it hard enough, though the effort involved can be astronomical. For tehm the physical circle is a whole lot less literally nessesary and a whole lot more mental crutch to visualize the circle and energy flows more accurately. In SmF harry talks about how he can image basically any parts of the circle he needs to, just with more effort, and in FM he creates a circle by swinging a glowing neckalace in a circle above his head really fast. On the flip side any time we've seen him go full ritual on a spell he was nervious about, that's him adding extra props to aid the mental construct.
Im not sure but I think that theoretically he could even imagine Iron to get the anti-Fae properties without actually having any. Given how the battle of wills with Mother Winter Went down, and a WOJ that states there is no upper limit to magic, I think it's possible, but I couldnt say for sure whether it could be done without Soulfire.