Why are these 2 Rituals? It's not the level?
Just the quick question: is the goal to destroy a city, to destroy its inhabitants, or both?
If you're going with rituals anyways, Instant duration rituals remove the need for penetration, often for less vis (because the duration is negligible), and they aren't subject to PeVi dispelling. Drop enough bricks/boulders above a city and little will survive.
Good question, I thought that that deserved to be rituals. Month long gale force winds over a city just seems appropriate.
I guess both really, although killing just the people would also work just fine.
Ooooh, good call, that would certainly work for the Deluge of the Million Boulders, not so much the others as they require an ongoing effect to really get the best out of them.
In one of our sagas, Kiev was almost completely depopulated in the fall of 1240 by an enraged maga through a few dozen castings of Octavia's one hundred legions (a Cr(Re)An level 35 Formulaic spell). The maga and her sodales ultimately managed to cover the whole thing up, and have the destruction go down in history as carried out by the Mongols (who actually only arrived at the spot a few weeks later).
I personally feel a target boundary creo mentem to enrage the citizens and thus cause a grand riot is an effective method.
PeCo, base 30, target boundary. The direct approach.
Going as low as I can while keeping Arcane Connection range:
Curse of the Starved Town
Perdo Herbam(Animal) 40 (ritual)
R: Arc, T: Group, D: Mom
All the food within a targeted region is spoiled, up to a total of a season's rations for 10,000 people, or a month's for 30,000. If cast in winter it is unlikely that any city will be able to survive the disruption. Even in cities with significant food reserves this will result in severe health difficulties, starvation and likely rioting. It is unlikely that the city will survive the impact of such an event.
(Base 3 +4 arcane, +2 group, +3 size)
The Buried Settlement
Creo Terram 40 (ritual)
Creates 100,000,000 cubic paces of mud, blanketing a region 8,000 paces in diameter to a depth of a 3 feet. A city thus afflicted becomes essentially uninhabitable.
(Base 1 +4 arcane, +7 size)
They don't look like they need to be ritual to me. If a magus wanted to destroy a city it is only a matter of time, and consequence.
Something like The Liquid Earth with increased effect size will do just as well.
The Silent Death
PeAu40 (ritual)
R: Arc, D: Sun, T: Part
A 2 Mile wide dome of air is destroyed for the duration suffocating those encompassed. When the spell expires a massive gale force wind will rush in to replace it causing considerable damage.
(Base 3, +3 sight, +2 Sun, +1 Part, +4 size)
An extrapolation of an effect mentioned on ArM pg 56.
That would be page 86...
Ack... Dang Number Pad. 
I only know because I was putting it on my list of spells to invent for my weather maga...
Ignem
CrIg50 Obvious Option of Oblivion
Base 15, +1 Touch, +2 Sun, +4 Size
Ignites a blaze inflicting +20 damage the size of ten-thousand bonfires that burns constantly for the duration, even after everything is turned to ash.
You could make it a level 40 momentary ritual, with a huge blaze that goes poof and hope everything catches.
It may need some fine tuning. And to be cast by a wizard immune to heat & flames.
Actually PeAu base 3 (make air stuffy and poor for breathing) duration inst target boundary (entire city) might not completely destroy the city, but given how long it will take for fresh air to reach the center it will certainly be devastating...
It's a momentary ritual; the fire is natural. That means that things will catch, because the fire naturally continues burning if there's fuel.
It's a momentary ritual; the fire is natural. That means that things might catch fire, if sufficiently flammable, there's sufficient air and all that.
It won't be resisted by magic resistance, but there are all sorts of reason it might not catch.
It's a medieval city. There's wooden buildings everywhere. Unless it's just flooded there's dry wood on the inside of those buildings.
Dry wood immersed in a bonfire will burn, thus feeding the fire. Everyone knows that.
So no, I do mean things will catch, because as long as you're targeting a city it's inevitable.
If it was a momentary non-ritual fire things might not catch, because the fire might not have time to cause smaller, natural, fires. But as a momentary ritual, cast in an area that has both vital air (it's a city, meaning people live there, so it must have vital air) and dry wood it will continue to burn until something puts it out, it runs out of fuel or it destroys the vitality of the air.
If it runs out of fuel: Congrats, you've just burned a city to the ground.
If it runs out of vital air: Congrats, you've just suffocated every living thing in the city.
If something puts it out... that something is really impressive.
Wood isn't garanteed to catch fire if exposed to flame for a few heartbeats (6 secons, the length of a combat round).
Certainly not good oak (in my limited experience).
Admittedly thatched houses are very likely to catch fire, but as I understand the risks is generally overestimated.
That's an interesting argument.