"Once upon a time a nation decided to end the threat of the tarrasque once and for all. An army was assembled, led by the greatest heroes of the age. Most importantly, a number of powerful magical weapons were created for the battle. The monster was lured into a tight canyon and the battle began.
"At terrible cost, the tarrasque was defeated. But not slain. It was impaled by fourteen Immovable Harpoons (like an immovable rod, but spikey), each attached to a thick adamantine chain sunk deep into the canyon walls by magic. The tarrasque was restrained.
"A fortress was built around the tarrasque, to watch over it. Every day it's watchers hack away at the tarrasque with powerful magic weapons to keep it weakened in case of escape. Even so, there are casualties as they misjudge its reach, or as it's angry thrashing causes rockfalls.
"Of course, being a powerful magical crearture, the tarrasque's blood, flesh and other body parts have certain useful properties. A side effect of keeping the tarrasque imprisoned like this was a neverending supply of powerful magical components. A city grew up around the fortress to house the various wizards, scholars and alchemists that came to exploit the tarrasque's bounty. Eventually, it was almost as if the neverending stream of tarrasque blood, flesh and bone was more important than imprisoning the beast itself."
I'm picturing decadant nobles made immortal by their continuued consumption of tarrasque flesh. Warrior-butchers wielding vorpal greatswords to hack away at the tarrasque and channels cut into the stone underneath the beast to channel the valuable blood away. The tarrasque's distant screams and roars would be a continuous background noise for the people in the city, with "tarrasque-quakes" common. Almost an industry of ludicrously expensive magic items crafted from its body parts - tarrasquehide armour, tarrasquebone spears and potions and other alchemical miscellania of course.
You could play up the creepiness of the whole thing, maybe eating the flesh and blood of the tarrasque has unwanted side-effects. I'm thinking of tarrasque blood being analogous to the spice from dune - in this city the blood's used in just about everything and it has unusual effects on the populous.
Even with the tarrasque mostly restrained, getting close isn't a good idea and there'll be pretty frequent casualties amongst the butchers. Because of its reflective carapace, mnagic is a no go so it has to be someone getting in close with a big knife. Every now and then the chains will need to be re-planted to make sure they've not been loosened by the tarrasque's thrashing about - what fun that'll be.
And there's the whole hubris angle - maybe the pressure to cut away more and more of it lets it pull free of one or more of the immovable harpoons. And an inevitable tarrasque-worshipping cult that is covertly planning to free their god.
And if you want to play up the "tarrasque as force of nature" thing, maybe its imprisonment is throwing the natural order of things out of whack. The tarrasque is a necessary part of the ecosystem and plays "natural predator" to something really nasty. Without the tarrasque killing off the nasties every X years they've had time to grow into their adult, even nastier form.
I mean, come on; a fortress built around a chained godzilla who's constantly being butchered is dripping with adventure hooks and just plain cool.
