My answer has too parts. Part One explains how some fantasy dragons are large and deadly enough to destroy cities. Part Two discusses ways to create a science fiction monster as large and deadly as some fantasy dragons.
PART ONE: Why should a dragon be unable to destroy a city?
As you may remember, the first chapter of The Hobbit features the dwarves telling how the dragon, Smaug the Golden, destroyed the human town of Dale and then the Dwarf kingdom under the Mountain in a single night of horror.
Later in the book Smaug sticks his head against a door because it is too large to fit inside the doorway and sends flames a long distance up the passageway after the fleeing Bilbo. As a child I compared the width of Smaug's head with the total length of Smaug's body in Tolkien's picture "Conversation with Smaug" and calculated that Smaug must be at least 200 feet long.
At that size Smaug might be able to simply step over the walls of a walled city to get inside. Or smaug might swing his mighty tail, which he used to smash up a mountainside in the book, to knock down a section of city wall. Or he might smash through a gateway, enlarging the opening if necessary as he pushed through.
Once inside a city, Smaug might roam the streets like The Beast from 20,000 Fathoms (1953), Godzilla (1954), The Giant Behemoth (1959), or Gorgo (1961)'s mother, etc., etc., no doubt blasting a lot of people with his flamethrower breath.
When Smaug came to a square he might turn around rapidly in the middle of it, swinging his hundred foot long tail against all the buildings around it, smashing them and their occupants.
Of course, Smaug would be unlikely to kill all the inhabitants of a city in one single attack. If Smaug burst in through the East Gate, for example, thousands of terrified civilians would probably stream out of the city through the North Gate, the West Gate, and the South Gate. Smaug would have to chase down those separate streams of fugitives with superior speed to ensure that the majority of the city's occupants were exterminated.
Of course nobody knows how much larger and more powerful than Smaug the largest and most powerful dragons were in Tolkien's legendarium.
So obviously in Tolkien's legendarium dragons could be large and dangerous enough to destroy cities, and sometimes did destroy cities and kingdoms.
That may be good for Tolkienian fantasy dragons, but what about somewhat plausible dragons in a more or less science ficition story?
PART TWO: Plausible city wrecking dragon-like monsters in science fiction.
If your dragons are large flying creatures perhaps a few times the size of the largest prehistoric flying birds and flying reptiles, perhaps you can have giant dragon-like creatures who walk and can't fly, much like the first generations of dragons in Tolkien's legendarium, where the first flying dragons appeared centuries after Glaurung the Golden, father of Dragons.
So you could call both the smaller flying creatures and the much vaster walking creatures dragons, being flying dragons and land dragons. Or you can call only the flying creatures dragons and call the walking monsters something else.
What is the maximum size of a monster that walks on land in a science fiction story? Presumably the world would have a surface gravity similar to that of Earth.
And such a question has been asked here at least once before:
Can a land-based organism get to be at least 100 ft. in length and still be plausible?1
In my answer to that I quote a Wikipedia list of the ten species of sauropod dinosaurs with the longest estimated length ranges.
Each of the species listed has an estimated total length range with a maximum length of at least 32 meters or 102 feet, and up to a maximum estimated total length of 39.7 meters or 130 feet. So the best estimates of paleontologists are that there have been at least ten species that walked the Earth with maximum total lengths of over 100 feet.
So my calculation that Smaug could be over 200 feet long doesn't make Smaug seem so impossibly long compared to known species from Earth's past.
But that's not all!
Two fossils were discovered indicated that the dinosaurs they came from might have been much larger than any the dinosaurs on the list. Tragically, those fossils are now lost. But Amphicoelias fragillimus (also called Maraapunisaurus fragilliumus), and Bruhathkayosaurus mathleyi might have been much longer than any of the dinosaurs on the Wikipedia list. Read about them and weep.
Note that the most extreme,though not most plausible, weight estimates for Bruhathkayosaurus were over 200 tons, and the longest suggested length for Amphicoelias fragillimus is 58 meters or 190 feet.
Since Tolkien's drawings of Smaug and other dragons give them very slender body proportions, they would weigh much less than Sauropod dinosaurs with the same body lengths.
Anyway, the sizes of the largest known sauropod dinosaurs indicate that natural evolution on an Earth-like planet could produce land animals over 100 feet long, and possibly almost 200 feet long, since it did so on Earth. And possibly natural evolution on some other Earth-like planet could even produce land animals over 200 feet long.
Natural evolution on an Earth-like planet can result in land animals weighing over 22 short tons. Wikipedia's list of the estimated weight ranges of the ten heaviest Sauropodmorph dinosaurs:
Has 22 short tons as the lowest estimated weight of the lightest dinosaur on the list, and 50 short tons as the lowest estimated weight of the heaviest dinosaur on the list, with 65 short tons as the highest estimated weight of the lightest dinosaur on the list, and 99 short tons as the highest estimated weight of the heaviest dinosaur on the list.
So it wouldn't be too speculative to imagine that possibly a land animal on Earth could achieve weights up to 100 tons.
The most extreme, though not most plausible, weight estimate for Bruhathkayosaurus was 175 to 220 tons. I suppose that there is a remote possibility, however unlikely, that Bruhathkayosaurus might have actually weighed as much as 220 tons. But I wouldn't count on that estimate as proving that 200 ton land animals are possible.
I note that the OP includes this specification for his Somethin' Else:
Somethin' Else is artificial (as in not the product of natural selection), and thus, they will be optimized for their chosen method of genocide.
So Somethin' Else, much more terrible than the relatively small flying dragons in this world, can be artificial, and thus designed by some intelligence to destroy cities and wipe out their populations.
In The Lord of the Rings it was mentioned that the first Dark Lord, Morgoth, created the first orcs and trolls thousands of years earlier during the Elder Days, modifying and perverting other organisms into those twisted creatures. Treebeard mentioned that to Merry and Pippin in The Two Towers, and the appendixes of The Return of the King mentioned that in the later Third Age Sauron modified and "improved" the original trolls and orcs.
In Jack Vance's The Killing Machine (1964), second of the Demon Princes series, the notorious criminal Kokor Hekkus has found the lost colony planet of Thambar and likes to play at being a warlord there, importing advanced technology to give him an unfair advantage. So it is possible that Vance got that idea from The Lord of the Rings and maybe Kokor Hekkus had read The Lord of the Rings thousands of years in the future and is playing at being Morgoth, an off planet invader with powers greater than those of the natives.
In The Killing Machine (1964) Kokor Hekkus commissions a war machine to take to Thambar, a machine that looks like a giant centipede. So Kokor Hekkus, like Morgoth, constructed giant monsters for use in battle, even though in this case the "monster" was a machine that the people of Thambar would only mistake for a monster.
So in your story there could be giant artificial monsters that attack cities, which might resemble wingless dragons, or super giant sauropod dinosaurs, or Godzilla, or other movie monsters who wreck entire cities.
These monsters might be giant machines built to resemble monsters, or cyborgs, biological organisms with artificial mechanical parts to increase their capabilities.
So some intelligence in your story who is playing at being Morgoth or Kokor Hekkus might import giant animals from other planets, and might genetically modify them in various ways, and might implant various devices into their bodies, and might train them or control them by electrodes inserted into their brains.
The have been a number of questions on this site about what substances giant animals might use in their bones and muscles to be much stronger than natural and thus be able to live and function at their vast sizes. And I think there might have been questions about surgically inserting metal bones, for example, into giant people and animals to enable them to function at sizes much larger than natural animals could.
So you might want to look for such questions and their answers.