The city of Troyes is primarily known for creating three things: champagne, knitting, and weighing gold. Two of these are medieval.
Let's start with the one that isn't medieval: the drink champagne. It is named for the region, Champagne, which has been called that since the Middle Ages (Campania in Latin). In the twelfth century the counts of Champagne, who controlled more territory directly than did the kings of France, made Troyes their capital. Troyes, along with several other cities of the region, held major trade fairs where wool from northern England, spices from the far East, and everything in between was traded.
But what does this have to do with the bubbly drink, you say? Sparkling wine was invented in the sixteenth or seventeenth century, traditionally by the monk Dom Pérignon (in a monastery outside of Troyes) although probably several different people came up with it independently (and accidentally). Modern bottling methods were required to hold in all those bubbles--medieval wine was kept in barrels which would have exploded. By the way, it's only really champagne if made in Champagne.
Let's get back to the trade fairs. Every city had its own system of weights and measures--it would have hurt their civic pride to use somebody else's measures. The weighing system of Troyes became known, not surprisingly, as troy weight. Although most of the world uses the metric system (grams and kilograms), and the British and Americans use British (also known as avoirdupois) pounds with 16 ounces to the pound, gold is still specified in troy weight. There are 12, rather than 16, troy ounces in a troy pound. Silver and gem stones are also often specified in troy ounces.
The city of Troyes also claims to be the home of knitting. In weaving, parallel threads, all running one direction, have another set of parallel threads woven through them perpendicularly. Weaving requires some sort of loom. Knitting, on the other hand, is based on a single thread (or yarn) looped and knotted around itself, usually using knitting needles. There are a few indications of some version of knitting going back to antiquity, but medieval cloth was woven, not knitted. But in the late Middle Ages knit objects begin to appear. The process is especially good for things like stockings and gloves that need flexibility, which knit fabric does much better than woven. The city of Troyes developed a true knitting industry by the late Middle Ages. The French word for knit (tricot) derives from the Latin for Troyes (Trecassium). (The English word knit is related to the word knot.)
And then there's Chrétien de Troyes, who essentially invented stories of Arthur, Lancelot, and the Round Table in the late twelfth century. Visit Troyes. It's a delightful city.