Wards are one of those things where the basic rules are fairly easy to understand, but are unfortunately a mite vague in the details:
A. Wards do indeed need to penetrate - so your level 30 Circular Ward vs. Demons needs to have an additional 20 levels of success in order to contain a lvl 20 demon. So, the caster needs to be able to have a casting total of 60 or higher to take full advantage of a lvl 30 ward. (it can still be cast with less penetration, though - possibly by a magi who learned a high-level ward, and is hoping to grow into it over time.)
B. Wards work both directions - so a demon on the inside of a ward can't directly affect the circle, nor can a demon on the outside. "Warded things cannot act across the circle, no matter which side they are on, nor can they damage the circle, directly or indirectly. (AM5th, pg. 114, 'Magical Wards').
C. The general consensus is that wards are either spherical (most common), or else cylindrical, with the max height being debatable - some few have argued that they're infinitely tall, but most use something like "twice the width of the circle." For non-geometric wards (such as an Aegis Boundary effect) it's usually considered "as close to spherical as you can get, if you include the height of the actual boundary". So if your boundary is on a castle that has high walls, the actual 'bubble' part of the ward would protect the entire wall. Others just go straight up, and have a tabletop roof. Personally, I allow both styles in my games, as determined by the caster.
C. As such, the issue isn't whether or not they can damage the circle: they can't. The only question remains is "just how far does 'indirect' go in terms of protection?"
D. There does seem to be a general agreement that that a warded creature can't use any technique, in and of itself, to directly affect the circle - so, no slashing at it with its claws, and no using magic to levitate something across the boundary. The not-so-obvious questions, though, are the following:
a. can a demon throw something at the circle? In Aristotillean physics, a flying projectile is moved by the air, not by the thing that threw it. Thus, there would need to be an Arum component to avoid such issues.
b. can a demon outside a ward get something above the line of the circle, and then drop it? Because (again), according to Aristotillean physics, objects naturally drop due to their own accord. You'd need the appropriate Form (Terram/Aquam/Animal/whatever) to avoid this.
c. Can the demon call across the boundary (with perfectly ordinary, non-magical words) and try to convince someone to break the ward for them?
Personally, I would say that the first two are covered by "indirectly", while the third isn't. The basic argument is that "indirectly" has to have some meaning, and casting a spell on the ward still seems fairly direct. However, others have argued differently. To be consistent, I go with the idea of "last willful intent" - for any given object that may potentially cross the boundary, what was the last free will action in the chain that ultimately made the object move? If the ward protects against that, then it can't cross the boundary. (For scenarios in which no will is involved, the wards protect as-is: so a water ward will protect against rain, and a corpus ward will protect against a person both trying to cross, as well as if they were thrown across by no intent of their own.)
To use an extreme example: if a circle against natural beasts (ReAn) was hit by a flood, normally it would be washed away. However, if the flood was ultimately caused by a beaver tearing down its dam, then the circle would protect against the flood, as a natural beast was the most-recent actor in that causal chain. If a human being knocked the dam over though, then it wouldn't protect it.
F. What counts as "breaking" a ward? Obvously, actually facturing the circle counts. But what about just laying something across it? How about ceremonially breaking a permanent ward with a piece of chalk or a stick? It's not explicitly stated anywhere. Personally, I go with " semi-permanent circular wards can be temporarily suppressed with a piece of chalk, but only if you make a magic theory roll. Just dropping a stick on it won't work. However, temporary wards (ie, those drawn in the dirt or mud or with chalk) are very easy to disturb, so the stick may break it anyway."