Trapping a spirit, and Circle duration

[The_Fixer]

Hello everyone!

My ars-fu is a little rusty, and, sadly, I don't have the time to plunge back into my books (My Ars space nowadays is mostly when walking to/from work ), so here I am with a few questions.
I've read this topic, but I disagree with some points, so, if you'll pardon me, I'll begin by explaining my reasoning.
(I thought about using mighty necromantic magicks to raise forth the old topic, but I didn't want to confuse people. I'm still not sure what would have been best :-/ )

As it is, a Circular Ward against #### is D: Ring and T: Circle.
Creatures are unable to enter the circle, or harm those within.

Now, the definition of T: Circle is "the spell affects everything within a circle drawn by the magus at the time of casting, and ends if the circle is broken"

What does this mean?

First, it strikes me that, since the definition of T: Circle doesn't include it, by the rules, any demon can just erase the circle with its feet and then do whatever it wants. Maybe it should be errataed to add something like "You may design the spell to that it also affects the circle" (Putting this as an option so your CrIg spell doesn't burn the circle)

Second, at least for these spells, the duration works in a peculiar fashion.
It is not "the spell affects everything (which is) within a circle drawn by the magus at the time of casting" (meaning you could get into a circular ward, go out of it, and stay warded for as long as the ring is complete)
but "the spell affects everything within a circle (,which is) drawn by the magus at the time of casting" (Meaning you're only protected while inside the circle, and the circle must be drawn by the magus while casting the spell).

Now, can someone who wasn't into the circle when it was created enter it, and be protected? This sure looks like it, and logic says yes. Otherwise, a demon would just have to get something into the circle and then attack it physicaly to enter it.
So I'd say there's at least one interpretation in which T: Circle spells affect whatever is inside the target, whenever it is inside.

Which would be perfect for what I want: Create a circular ward which traps any creature that enters it.
For those who know it, the tv show Supernatural see various uses of these: They inscribe a circle, an angel/demon walks into it and is then prisoner.
There's also the whole summoner vibe of inscribing a circle that'll hold an entity, summoning it inside, while, for added protection, you stand in the middle of a circular ward. Even if protected, I personnally wouldn't want to summon an entity that wasn't contained: It may cause trouble before I Rego control it, and I may also botch.

You would think this to be simple: Just use the Ward guidelines.
Problem is, these protect the target, they don't ward the world against it.
So, in theory, if you inscribe a circular ward around a demon, it'll be warded against demons, but there's nothing warding demons against it. Likewise, it can use its powers outside the circle. But if it goes out of the circle, it won't be able to enter it again.

So I thought, "why not use a non-standard target akin to Circle, but working in reverse, like Target: Pentacle or something, which would protect whatever's outside of the circle from what's inside". This part would work, sure, but the drawback is huge, and obvious: I could cast a t:Pentacle spell to affect the whole world! So that's a no-go.

This, I came back to T: Circle, and thought about using another guideline. I seemed to remember examples of Spirit prisons in the books, but a cursory search only revealed "empowering prison of the declining spirit" (in tMRE and LoH), which isn't what I want. Maybe I'm just imagining things.
So I thought about using the base 15 guideline to control a creature of might, with an order hardwired into the spell being basically "stay nicely into this cirle, thanks"

What do you think about it? Did I miss something, or made a mistake?

Bonus question: I can't remember anything precluding the usage of D: Ring with targets other than Circle? It looks like an blatant can of worms (Cast a D: Ring, T: Ind ward, go out fight those demons!), so I must have forgotten something obvious, but if someone could tell me what?

And I had another question, but since I've written this over 2 weeks, it's fallen into the cracks

Thank you very much for your time and help

[Eufemia]

@The_Fixer

You would think this to be simple: Just use the Ward guidelines.
Problem is, these protect the target, they don't ward the world against it.
So, in theory, if you inscribe a circular ward around a demon, it'll be warded against demons, but there's nothing warding demons against it. Likewise, it can use its powers outside the circle. But if it goes out of the circle, it won't be able to enter it again.

Regarding this part, I'm not sure if I understood exactly what you are asking, but this is what the Magical Ward guidelines say:

Wards with a Circle target are of particular note. They prevent things warded against that are within the circle from leaving, and prevent things warded against that are outside from entering. Warded things cannot act across the circle, no matter which side they are on, nor can they damage the circle, directly or indirectly.

[Eufemia]

@The_Fixer

Bonus question: I can't remember anything precluding the usage of D: Ring with targets other than Circle? It looks like an blatant can of worms (Cast a D: Ring, T: Ind ward, go out fight those demons!), so I must have forgotten something obvious, but if someone could tell me what?

Regarding this, there should be no limitations precluding the usage of D:Ring. It it not explicitly said, but here what is said:

It is quite common to create such
spells with a duration of Ring, as then the ward
can be made to last indefinitely, with care.
However, this is not required, and the spell can
be created with any duration.

The spell guidelines for wards against
magical things are listed with a base range of
Touch, base duration of Ring, and base target
of Circle, because a ward with these parameters
wards against creatures with a Might equal to
or less than its level. The parameters of the
spell can be changed in the normal way, so that
a ward which only protected the magus (Range
Personal, Target Individual) for a duration of
Moon would have the same final level

[The_Fixer]

@Eufemia

Wards with a Circle target are of particular note. They prevent things warded against that are within the circle from leaving, and prevent things warded against that are outside from entering. Warded things cannot act across the circle, no matter which side they are on, nor can they damage the circle, directly or indirectly.

Thank you very much! This is exactly what I sought. Nice to see they thought about it, even if I feel quite dumb now I really should have remembered that.
So, this solves the problem nicely, since a non-standard target based on Circle would do the trick, and I've never had the chance to use one

For the bonus question, I'm sorry for the waste of time, I had a brain fart. I really must get some sleep.

Thank you very much for your time and replies