This is going to depend heavily on what part of Europe youre looking at, so the question is very, very broad, which is why I suspect you have not recieved answers yet. I will focus on Western Europe, especially Britain and France, with some examples also from Italy, since that is the area I know the most about. I will go over visual (and other sensory) clues, as well as some of the social changes that occurred during this timespan.

In 400AD the Migration Period had recently begun, and the Western Roman Empire was on the brink of collapse, but all three of the regions I mentioned would still be Roman at that time. While it would be very different from the late Republican/early Imperial peak, the Empire would be administering these areas, and in Britain, which held Imperial borders, one could expect to see military camps where legions would be stationed. Architecture would still be Roman, especially in the major cities, which would have a relatively cosmopolitan character compared to what was to come. In British, French, and Italian cities, and much of the Italian countryside, Latin would be spoken. In the countryside in Britain and France you would expect to hear a variety of Celtic languages. Christianity would be the official religion in all these places, and small basilicas would be visible in major cities, but more rustic regions, especially in Britain, would retain varying concentrations of paganism. Mithraism, a Persian-influenced mystery cult popular with the legions, was at its peak around this time, and Mithrea (natural or artificial caves where Mithraic worship took place) could be found anywhere Roman military officials or soldiers were common. However, in Britain especially, but across the Western Empire the beginning of a trend that would continuefor centuries had already begun, soecifically, deurbanization. Compared to even 50 years earlier many cities were beginning to depopulate, and even Rome itself, no longer the capital of the Western (the less powerful of the then two Roman empires) was significantly diminished from its peak centuries earlier.

The recession of Roman influence over the following centuries took different forms in different places. In Britain the Romans left for reasons that are still debated; I am of the school of thought that the Roman beaurocracy and infrastructure left them unable to effectively manage such distant holdings, leading to an essentially bloodless abandonment of the region before the Germanic migrations reached their shores. In Italy and France, Germanic tribes had arrived, and the Empire first tried to placate them by giving them land and a series of political concessions, which led to increased demands and finally widescale looting. Cities continued to depopulate, and what new construction there was was far less elaborate and focused almost entirely on keeping the barbarians out. Cities deep in the formerly peaceful Imperial territories required functional walls for the first time in centuries, which woukd be built, like the earliest castles, from earth and wood, and with no empire to protect them, people turned to local landholders, who might be clergy, native leaders, or sometimes settled 'barbarians' to protect them, in exchange for loyalty, taxes or labor, and military service. This is the birth of feudalism. Depending upon what culture was dominant in a oarticular area, succession on the death of these leaders took different forms. In some cases lands would be divided amongst his sons, while other tribes would hold elections in which the members of the budding landed nobility selected from amongst themselves their new ruler. Areas where Latin had penetrated heavily saw increasing divergence in the dialects of vulgar Latin due to decreased contact between communities, later diverging to languages like Old French, which often existed alongside Germanic languages brought by the new conquerors like Frankish in France, Gothic and Lombard in Italy, or Anglo-Saxon, and a few centuries later Old Norse in Britain. Christianity was at this point the dominant religion in the regions I am addressing, with the exception of Danish and Jutish territories in Northern England.

Eventually most of the Germanic tribes who had precipitated the fall of Rome settled and feudalism, in its different forms, settled with it and was systematicized and enshrined in law. This is the mature version of fedualism most people associate with the word. Titles both landed and honorary became hereditary, with the first son inheriting the bulk, or sometimes the totality, of his father's rights. This led to the division and conflict between the royal and noble classes (although there was still significant crossover between the two). The market fairs held in population centers meant that larger settlements again were more important economically than militarily, and their cosmopolitan character began to return as merchants travelled great distances, relative to what was safe or practical a couole centuries earlier, to sell the products of their home and purchase materials and products to sell back home. Trades began to develop into the guild systems in the market towns, and the burger class appeared in the cities of northern France and the Low Countries. A few of these cities had the distinction in this period also to become home to the first universities. Similar developments occurred in Italy. The Romance Languages had fully individuated by this point. Churches and castles became significantly more elaborate and permanent; along with city walls and other large and permanent structures, they were built mostly from stone going forward. The Norman conquest of England brought a French aristocracy to that country, which lagged somewhat in development before that point, for a number of reasons.

This brings us to the peak of the High Middle Ages as the 12th century dawns. I know a lot less after this point, so Ill leave other regions and the last two centuries of the period you asked about to other members. I gave a very basic overview and glossed over or even neglected completely a lot of material. Please let me know if you have any specific questions.

EDIT: another point I neglected in my initial answer was the increase in domestic European agriculture during the period I discussed. Initiated by the collapse of the Roman trade network and exacerbated by the Islamic conquest of North Africa was the loss of Europe's ability to rely on what was once the breadbasket of the empire. This augmented the deurbanization of formerly Roman regions, as populations who had lived in cities and survived on African and Near Eastern grain were forced to take to the fields and produce food for their own communities.