Use the adjective medieval to describe something from the Middle Ages, or something so backwards that it might as well be from the Middle Ages. A teacher once rapped your knuckles with a ruler as punishment? That’s practically medieval!
Medieval can also have positive connotations, since the Middle Ages are remembered as a time of knights and courtly love, when chivalry and honor were highly valued. If a guy doesn’t mind getting his feet wet lifting his girlfriend over a big puddle, that’s medieval in a nice way. However, if he then dies of the bubonic plague, that’s medieval in a less nice way.