Since you can't effectively work the big fields without animal power, you need to turn to the next best source of food - the forest.
The concept is called Forest Farming (or in recent years, Food Forest) and was practiced for centuries in the Americas and many other places outside of Europe. To be fair, it was mostly in hotter climates where the shade provided by the canopy was seen as a bonus. In most European contexts, it would be seen as a hindrance. Other pros apply. Forests, even when heavily managed, have huge potential for soil regeneration. They hold water in the environment and even out the adverse effects of the climate, like strong winds or cold snaps, that can destroy crops in open fields.
In a European context, you would want to maximize the "Edge of the forest" biomes, as it benefits fat-rich nuts like hazelnuts and sugar-rich berries, and attracts animals for hunting.
Also, predominantly oak, beech, and elm forests would gradually be replaced with food-bearing trees like walnuts, chestnuts, and various fruits.
European cultures would therefore rely on heavily managed forests, completely made up of edible plants complemented with patches of easily manageable grains, legumes, and vegetables. Since you don't need pastures and can't manage big fields, every free place where something can sprout would be forested to take advantage of less energy-dense food options.
Lower Population Density:
Population numbers would be lower, yet significant civilization centers would still emerge. Especially in places where other food sources are present, like seashores and major waterways Their size would be smaller than we are used to imagine and their character would be more akin to Celtic 'Oppidum' or early Slavic 'Hradisko'. People feeding those centers would be much more dispersed. This means that rural organization would be based more often on the family or clan as a base unit (as opposed to villages), and the difference between city folk and rural people (already significant in the Middle Ages) would be even more pronounced. Control of such a dispersed rural population would also limit the usefulness of the feudal system, since the lowest tier fealty (serf->lord) would be significantly harder to enforce.
Less Castles, More Brambles:
Organizational centers would be in cities. To remain relevant, cities would need to be centers of non-food production. It is possible they would be more specialized in certain industries than they were in our history they would be fortified and double as refuges.
Since all open plains were taken up by planting trees, traditional armies and open field battles have no place in warfare. Instead, smaller units would infiltrate enemy territory, targeting everyone they encounter to avoid guerrilla responses. The home population would then rapidly move to the "cleansed" land. Additionally, forests contribute to defenses. In medieval times, lines of forests near borders were sometimes clear-cut, and clearances were left to overgrow with brambles and other hard-to-trespass vegetation. Wild apple trees were often planted near roads because when cut down, their branches provided an effective barricade.
There Will Be No 'Medieval Europe':
Now, let's address the elephant in the room. 'Medieval Europe,' however vague and uncertain that term is, didn't just emerge out of nowhere. To get 'Medieval Europe,' you need the Mongol invasion, Umayyads in Spain and their struggle against the Frankish empire, Scandinavians going on Viking raids, Germanic tribes dismantling the Roman empire, and Avars and Magyars entering the Danubian basin. You need the migrations that pushed Germanic people into Celtic Europe to happen, and you need the Celts to come and form their culture. You need Mycenaeans and Minoans to emerge and be replaced by Greeks. You need Indo-Europeans to rise and come to Europe, replacing whatever pre-Bronze Age cultures were there at the time.
All of those events and civilizations were also shaped by the existence of animal husbandry. Without it, the world would be completely different.
How do you even arrive at Medieval Europe without domesticated animals?
Did they all die at one point? Then an era of famine and chaos would follow such an event.
Did it never occur to people to domesticate? Then you need to start reshaping the world long before the medieval era.
Were there never animals convenient for domestication? Well, aurochs, ibexes, or horses were often the prey for our ancestors. Without them around, maybe our primate predecessors would never have considered jumping down from the trees to hunt them and become, you know, human.