From this time on, two climbing plants have dominated Central Europe: Ivy and Grape Vines. While Ivy grew up without invitation, the breeding of grapevines was a deliberate facade greening of houses. Introduced by the Romans in the first millennium after Christ, grape wine was needed, especially as a sacramental offering for their cult, but also as a comforter for toil and trouble and - distilled- as a disinfectant and anesthetic for medicine. But there were always problems with the ripening of the berries. What was found in the ancient barrels often resembled acidic vinegar instead of drinkable wine. Because the native south vines lacked the warmth, and the German monks were trying to breed very early maturing varieties it made for sour grapes.
When grapevines were put on protective monastery walls, vineyard walls, or on house walls, they warmed up, matured better, and became very sweet indeed. Clearly, from such fruits (or from what children and a sweet toothed left on the vine) alcohol-rich grapes went immediately to the wine press. Wall greening was thus operated under quite a practical point of view...
Therefore, vines were always useful when buildings were greened in the middle ages. Everywhere such buildings were, viticulture followed! But today there are modern, fungus tolerant varieties to be preferred.
The question of whether the ivy on medieval castles was always there is up for debate. Tourists don't question it, they simply find it "beautiful." In any case, Ivy is always appropriate when large-scale greenings are desired on very old masonry.
Honeysuckle and especially Roses were detected within this period. Not our modern cultivated species, but the wild roses such as Rosa Canina, the 1000-year-old (Millenial) Rose at the cathedral in Hildesheim. Also espalier fruit and Hops were also interesting options to be found. Other plants can look very nice now, but they are only an encore of our modern times, not Middle Age originals. Also base greening comes into question.