Hmm.. looking at ReIg (for how "natural" fires transplanted are treated) there is both tremulous vault of the torch's flame, which refers to p. 181 for flammability though I don't see rules there which would indicate anything of the sort, or Leap of the fire, which indicates that a fire burns "naturally" while moving through the air independent of its fuel source...
It's a rego spell, they can essentially teleport things.
It says nothing about what happens during the journey, so there's no reason to assume that there's any meaningful time-period of journeying.
Oooh, I like this one.
Personally, IMS, this wouldn't result in a fine gravel, but rather in a pile of stone blocks (because muto can't destroy on its own) but it would definitely be highly effective 
Minor refinement: non-ritual spells like this probably aren't going to be cast in combat. As such, you can probably get away with designing them all for R:Touch and D:Concentration (as necessary, of course), and then using ReVi effects to extend the range and duration.
Unless the magi in question is hyper-specialized, it would probably be easier to learn all of those three spells separately (the attack spell, the Intangible tunnel, and Maintain the Demanding Spell) that it would be to learn one large one.
Hm. Serf's Parma: can you cast ritual effects through non-ritual Intangible tunnels? I seem to recall that you have to do ritual MuVi effects, but I don't recall for ReVi.
EDIT - taking the ReVi effects to their logical conclusion, you can probably burn a city to the ground with a judicious use of a low-level, Sun duration watching ward, a "Palm of burning flame" effect, and a Sun-duration, sight-based intangible tunnel. You'd just have to cast them all hundreds of times apiece: each combo would 1) Delay the spell for a few hours (say, until Noon), 2) target the roof of a given house, and 3) set that roof on fire.
Of course, this option runs into logistical issues - among them "how do you target each individual house?" and "how long can your magi do this before loosing mental focus" and "this is going to take a few hours - what if he gets noticed?"
If the city is walled, there are a number of techniques you can use with the Warding virtue from HoH:S - basically, any circle/ring spell can be cast on the whole city, simply by walking up to the city gates and marking it.
Even if the city isn't walled, you can still use the same virtue to cast circle/ring wards on the city, simply by walking the boundary - assuming you have some way to mark the boundary that won't get knocked over. (You have to physically mark it via some sort of caern, or sigil, or something - each within sight of the other.)
Warding a city against herbam (most food is herbam) is certainly a good way to destroy it. Either preventing the food from moving it or spoilong it as it enters the city. Nice one
I suppose one way to look at it is "how many low-level effects, if pushed out to a large enough size and/or duration, will cause mass panic and/or destroy infrastructure?" And in flipping through the book, I think the answer is "quite a lot".
A variant of this may have already been mentioned, but the MuAu general guideline "transform air into poison" is probably the lowest-level effect - although making it at lvl 1, for a d10+1 damage (per turn, if I read it correctly) is probably a bit munchkin.
Cloudkill: (MuAu 5) Transforms the air within a city into a noxious vapor, slowly killing all the inhabitants over the course of an hour. (General effect lvl 1: +1 damage per turn, Touch +1, Conc +1, Group +1, Size +1).
Most magi could probably spont that.
Although note that "boundary +4" just means "Area 100 paces in diameter" (AM5th, pg. 113) - so most of these spells will need at least a +1 or +2 magnitude to affect a city.
Auram being the main exception of course, with it's base individual size being 100 paces across. (So you can probably get away with "Group" for most medieval cities - which were pretty small, I thought.)
Most of the spells that 'slow kill' cities will run into divine intervention. There will be enough people with enough Faith Points that somebody will either successfully petition either a saint or get a direct miracle from God. Supernatural assault makes it easier to petition for intervention as well.
You can also run into square cube law issues depending on how you judge size increases. If your talking area a boundary 1000 paces across isn't 10x the size of a standard Boundary it 100(so a +2 not a +1). If your assuming volume is relevant and increases equally then it's 1000x bigger(+3).
I don't see why "slow kill" should be any different than "fast kill". Ars Magica is not a game where you have Michael the Archangel tell Saint Peter "Yeah, that terrorist attack on Jerusalem caught us pretty much flat-footed. It was over before the emergency prayers could make it through all the red tape. Man, the Boss is really upset."
maine75man, take in mind that creo spells to create fire must be individual or group targets, you can't use boundary with a creo effect to create something (although you could use it with a boundary to heal stuff of course).
IMS we are faced with developed cities that are not only warded but also contain many spellcasting types. Slow kill spells are not going to get the same level of killage (new word) as they would in mythic Europe. That said, I love the MuTe stone to sand spells. However, I have a query about them. If you muto'd a castle into sand, when the spell wears off, would the castle be little more than stone chunks/gravel, or would it revert to being a castle? Surely to destroy the castle, which turning it into gravel would surely do, requires perdo. You are making it worse. I've not actually encounter this in game yet but i did consider the same with a muto spell to turn a sword to sand, when the spell wore off would the sword now be a pile of iron in whatever shape it fell into while sand, or would it turn back into a sword. I'm kinda thinking it might turn back into a sword unless you add a perdo req. Thoughts?
The spells I wrote up use the CrIg Ignite guidelines so I don't think of them as actually creating a totally new thing. I think of igniting like healing. The target is not the healing itself but the body being healed. The target is not the ignition it's the flammable object that ignites. Therefore I think Boundary should be applicable to these spells.
I would also consider Boundary appropriator for many of Types of Creo eefects. A couple of others that come to mind. CrIg spells that heat objects directly or CrMe spells that put thoughts in minds.
I think a magus can design an effect to do either; subject to the whims of the caster.
i.e. It should cause the sand castle to collapse, which is then rubble when it reverts to it's normal form. However a MuCo/Au effect into air does not have the same effect when the spell ends; so there is precedent to say it does work both ways.
I don't think Perdo is required for your example, but might be in other more complex effects.
Muto can (imho) be used to alter the form of something, take advantage of the inheritance properties of that new form, and revert back to either the current shape, or the old shape. I'd argue that if the manipulation of the new form is magical or complex then a Rego req is needed though.
So a "sword to sand" is subject to the whims of the caster, but the caster can either have that sword back, or just a lump of metal. If the caster wants any other shape they need to do something else to get that way.
As an example a big stone block might be covered to sand, divided into smaller piles, then the effect cancelled. The result is many small lumps of rock.
would make for an interesting and entertaining side-story to see that play out a little. Gave me a laugh, thank you.
My thought is: You're not mutoing the castle, you're mutoing every stone object in the castle, including the stones in the wall.
So while yes, the objects mutoed return to their form at the end of the effect, their relative position has changed. So you don't end up with gravel, you end up with a pile of stones, each of which is one of the stones from the wall.
Key element: A perdo spell would weaken your theoretical castle's stone, making it easier to smash.
Gravel made from a castle would still be hellaciously tough, even if it's already in small bits. You're not making the STONE, which is what the muto targets, worse, you're changing the arrangement.
Creo terram to make a castle merely sets an arrangement of stone. Same with a rego terram to craft a castle.
Perdo terram causes the stone to crack and weaken until it can no longer support it's own weight, then nature takes it's course.
Intelego terram will only give you information on the stone, not the castle.
Thus, a muto terram to turn it into sand would simply rearrange the stone once the muto wears off. Although since stone turns into sand over time, it's technically rego since it's a natural process.
According to the rules (and in large respect the biblical paradigm), somebody has to call for Divine aid before it happens. By the rules, it has a fairly low success rate unless someone is really righteous (high True Faith score) is doing the calling. The high level of effect needed to stop the 'big spells' will be very difficult for them to achieve, essentially requiring Ceremony, which takes time to set up (assuming you have a holy group in the town). Another alternative is repeated calls for aid, which again takes time.
A very fast disaster may well leave no room for the faithful to even call for help (though virtues like Premonitions and Visions may well change that). The Divine does not seem to be 'proactive' in stopping these things, however - plenty of horrible things happen in Mythic Europe, and it's only when the faithful ask for help that they're likely to get it. The longer something takes to tear a town apart, the more likely people are going to successfully entreat the Divine.
It's far more likely that people with Premonitions or Visions will receive warning from the Divine and efforts to stop the 'big ritual' will take place.
WRT Actually doing these sorts of spells in ME, I thought the book answer was that the Divine DOES pro-actively get involved - which is one of the reasons why magi don't do it.
Re: How to destroy something with Muto - from what I recall of the last time we had this go-around, the general consensus was that muto by itself doesn't destroy things, but changes made to it while transformed to remain. In fact, this is kind of the point of muto - changing stone into clay in order to mould it, for example. So transforming a castle into water won't destroy it - rather, gravity pulling the water to the ground and splashing it all around will. Once the spell ends, you'll end up with castle splashed all around the countryside - probably in the form of gravel and chunks of wood.
Certainly there's always going to be the fear that the Divine may well smack you down, but the Divine does allow terrible things to happen to Mythic Europe. Wars and atrocities still happen, famines and plagues still occur. Not all the plans of the Infernal/Faerie are stopped by God, so why should all the plans of mortal magi?
The Order of Hermes (as a whole) is smart enough to realize that they couldn't withstand a concerted effort by the ruling class of Europe to get rid of them - at least not without undertaking the kind of organization and planning that only the Tremere seem to involve themselves in. Even if they won, the loss of vis sites and casualties would make it a Pyrrhic victory at best. There are multiple examples of mass destruction spells in the core rulebook; even if they don't get used often, somebody used them at some point and if the Divine auto-cancelled them magi wouldn't bother keeping them around.
Mankind wouldn't have much free will if God stepped in every time someone did something terrible. Cain wasn't stopped from killing Abel.