The basic farm tools that are known from the Medieval period are the ax, scythe, flail, shears, harrow, sickle, hayfork, spade, moulboard plow, wheeled plow, rake and winnowing basket. Versions of these same tools are still in use around the world where non-mechanized agriculture is practiced. The moulboard plow was guided by a soil-flipping sled and used on heavy clay soil; the more primitive wheeled plow was used on sandy or loose soil.
By the time of the Norman conquest in 1066, Europe was divided into at least 9,000 manors, or aristocratic estates. Each manor was a unit of economic production, with peasants paying tribute to lords in the form of agricultural produce. Fear of marauders and the patchwork of sovereignties meant peasants farmed on the same land over and over, with little resilience in the face of weather changes or pestilence and repeated exhaustion of soils. Improvements in the plow that allowed it to be pulled by multiple draft animals opened more land, and made possible a three-way crop rotation that allowed soil to rest and increased general output.
Between the fifth century and the eighth century, the use of the heavy (moulboard) plow expanded from Eastern into Western Europe. Until the eighth century, yokes for draft animals were affixed to the necks of the animals, choking them if they pulled too hard. In the eighth century, a breast-mounted yoke was developed that vastly increased the pulling power of the animals, now pulling heavy plows. By the 11th century, Europe saw the whippletree, a rig that allowed multiple animals--wearing breast-mounted yokes--to be attached to the same plow.
Horses eventually replaced oxen as draft animals because horses had better endurance and pulled faster. Monks had experimented with selective breeding and larger horses were coming onto the scene as the breast-harness appeared. The other invention that improved equine performance happened around 900 A.D., and that was the horseshoe. Horseshoes prevented horses' hooves from splitting in the soggy ground. While not a tool per se, the horseshoe became a force multiplier for the breast-harness, the moulboard plow and the whippletree.