My answer (don't have the book handy to confirm): Circles, like all thaumaturgy, rely on setting up symbols to channel power. If those symbols are disrupted the spell is weakened and may fail. In general, ward type symbols are protected from one side by the ward itself. The other side of the ward is open (usually).
A simple circle with a rating equal to Lore might be disrupted by anything breaking the circle. A more extensive ward might have several keystones, runes, or other symbols to break for complete failure.