The circle is affected by CAT differences, so Traces and other lower-level sins may find their attacks on it less effective, Risky, Hard, or eventually (with a CAT difference of 3 or more) impossible.
Prison mentions that it only impedes Sins, but it may be worth asking whether it can be partially or fully effective against other psychic creatures like drifters, human infested, bound sins, or exorcists with sin marks.
Prison doesn't make mention of knowing the status of the circle when unobserved by the summoner, but we think it may be possible. This is probably best done with a Psyche roll beforehand to know whether the user has a connection to it, or with a secret roll by the Admin.