Explain the key features of medieval Europe and how some of these features began to unravel by the end of the 15th century and the beginning of the 16th century. To what extent was the 15th century a turning point in history?

With the late fifteenth century you'd be looking at Europe during the Renaissance and the Age of Exploration. In any case, there's an additional degree of difficulty when factoring in the sheer timescale the Middle Ages represents, and how it can be further subdivided into by periodization (Early Middle Ages, High Middle Ages, and Late Middle Ages). Culture and social trends evolve, change and shift across time, and this isn't getting into the vast degree of regionalization that creates so many particularities in European cultures.
We tend to associate Medieval society with certain things however. The fall of the Roman Empire created fragmentation (even if there were large empires like the Carolingians) and from this fragmentation you see the rise of the feudal structure. This was a primarily agricultural society, where power tended to be held by local landholders (the Feudal elite), and over time as trade revived, so too did towns begin to grow and turn into cities. In addition we can mention that the Church, which had deep roots in the Roman past, was the strongest and most powerful institution in the Medieval World. As you can see, there was a lot of evolution within the Middle Ages themselves. It was a vast span of time, stretching from the fall of Western Rome all the way through to the start of the Renaissance, and a lot happened in that time.
The late 15th century and early 16th is certainly a significant moment of transformation. For one thing, there's the Italian Wars, which itself covered a significant span of time (1494-1559) and would devastate Italy, resulting in a shift in power away from the Italian City States, where the Renaissance began, and further north into Western Europe. Additionally, there was the Spanish Reconquista (which drove the Muslims out of Iberia) and the rise of Spain, as well as the discovery of the New World. The discovery of the Americas is a particularly noticeable turning point in European history, as it heralded the creation of Empire and the rise of the Triangle Trade. All in all, when looking at the era in question, we can observe power centers shifting, and also see European monarchies continuing to strengthen their power, which would eventually culminate with the Age of Absolutism.