In the Middle Ages they knew next to nothing about Achilles. Homer was completely lost in the west during the Middle Ages, though he was well-known to Byzantine scholars--the Florentine editio princeps of Homer was not published until 1488, and until then Homer was only known through half-assed Latin summaries like the Ilias Latina. The epic poet par excellence for medieval scholars was, without question, Virgil. Already in late antiquity Virgil had begun to take on mystical properties, and beginning in the 4th Century the Fourth Eclogue was already being interpreted as a prediction of the birth of Christ, leading to the folk-tradition that Virgil could be used to predict the future (by opening a codex of the Aeneid to a random page and reading the first line that your eye fell on), that Virgil had been a magician, and that Virgil would have been a Christian had he lived a little longer (hence his ability to travel freely through Hell and Purgatory in Dante--there's a tradition that Paul wept at Virgil's tomb because he was just a little too late to convert him, which medieval scholars thought Virgil surely would have accepted). As a result of Virgil's popularity (as close to sainthood as I think a pagan can get) and the complete loss of Homer in the west the knowledge of the Trojan Cycle among western medieval scholars was pretty much totally limited to Virgil and whatever Latin references to Homer they could get. Not surprisingly this resulted in a rather skewed understanding of what's in Homer, with the Trojans generally taking the forefront, in imitation of the Roman claim of Trojan ancestry. Various peoples claimed, in the wake of the Roman withdrawal, descent from Trojan heroes--the most famous is probably the medieval claim that Brutus the consular, who was given direct descent from Aeneas in the medieval versions, sailed to Britain and founded it (hence the name Britain, which sounds kinda-sorta-not-really-even-a-little like Brutus!). There were also other texts that influenced the elevation of the Trojans in the eyes of the medieval scholars. In late antiquity there arose something of a literary form of feigned eyewitness accounts from the Trojan War, the most famous of which were the stories of Dictys the Cretan and Dares the Phrygian, the "authors" of pseudo-autobiographical accounts. They have a number of features, including elimination of the interference of the gods that's so characteristic of Homer, and at times they contradict (or occasionally confirm, which can be helpful for establishing interpolations) Homer--their defining feature, though, is that they claim to be eyewitness accounts, which of course they are not. In the Middle Ages these tales, particularly that of Dares (on the Trojan side), were widely read, and they were considered by medieval scholars to be more authoritative than the Latin summaries of Homer they had, and even more authoritative than the actual Greek volumes of Homer that they got in the Renaissance--Dares and Dictys were, after all, eyewitnesses to these events, whereas the tradition held that Homer did not live until centuries later. As a result Dares and Virgil became the major sources for the entire Trojan Cycle, despite having little to do with it--we can also be sure that the Roman claim to Trojan descent had more than a little influence in the desire of many families to equip themselves with a Trojan pedigree.