Definition
Feudalism is defined as the legal and social system that evolved in Western Europe in the 8th and 9th centuries, in which vassals were protected and maintained by their lords, usually through the granting of fiefs, and were required to serve under them in war.
Historical context
After the fall of the Western Roman Empire, several succeeding kingdoms rose, such as the Kingdom of the Franks(In France), the Kingdom of the Visigoths(In Spain), the Kingdom of the Lombards(In Northern Italy), and the Anglo-Saxons Kingdoms of Britannia. The tribes that led these kingdoms had previously sustained themselves on raiding and pillaging wealthy settlements that belonged to the Roman Empire and its allies. Their style of life was tribal and migratory, rather than settled and urban as was the Roman style of living. Once the tribes began to settle in the cities and towns that they took from the Romans, they could no longer sustain themselves on raiding and pillaging, and under this new environment, a new system had to be created - later to be coined in the 17th century as Feudalism.
Birth and spread of Feudalism
However, because these tribes had no prior experience in urban governance, construction and tax-collecting, any attempt to establish a new central government was destined to fail. This also created a problem with a surplus of many trained soldiers which the kings could no longer pay with coin or treasures. Fertile lands and settlements were in great abundance, however - and kings and generals would soon reward their warriors with new territories instead in exchange for military obligations. This habit quickly resulted in the birth of a new social class - a heritable landed aristocracy with varying regional autonomies across the realms, who were known as vassals and were sworn directly to their king or a more powerful liege lord, who was sworn to the king. The feudal system was born out of this new environment.
The Christianization of the Western Kings and the introduction of the Church gave rise to a new state religion: Roman Catholicism. The Church, from Rome by the Pope, had sought to exert its influence over the vast lands of the former Western Roman Empire. Naturally, the Church found the present situation to be very convenient to establish its authority over the new autonomic vassals of the realms, as it granted clerics the ability to deal directly with every individual lord and not with the king alone. Therefore, with the Church's blessings, Feudalism became a universal system in Western Europe, and over time would spread to other Christianised domains in Central and Eastern Europe.
Serfdom
While the kings were now satisfied with guaranteed armies to be levied for their wars, the lords who ruled over the vast lands had to find the means to sustain such a military force, to begin with. This gave rise to the status of serfdom, which would replace the slavery of old. The serfs (also known to some as peasants) were essentially servants of their regional lord, who in exchange for housing and protection were obliged to pay much of their yearly products(depending on the profession of the serf) as a tribute, and could also be drafted as construction labourers or guard militia for an extended period of the year. This settlement tended to put the lords in great advantage before their serfs, as families of serfs would often fail to deliver the harsh tributes in time and would gradually indebt themselves to their overlords, to the extent of losing all their liberties for a life of service like slaves.
The new aristocrats
In addition to serfs, the lords now enjoyed the freedom to establish their personal military retinues, which were comprised of mounted knights(a small force of elite heavy cavalry) and semi-professional soldiers(mostly heavy infantry). Most of which originated from the wealthy middle-class(from the urban settlements known as towns and cities) or aristocratic families that could afford to be trained and equipped with arms and armour.
The lord, of course, benefited from a reliable source of cheap and replaceable labour to serve their means, whatever they may be. The position of the feudal lords was further entrenched by the mandate granted to them directly from the Catholic Church, which stated that God directly appointed all lawful feudal kings and lords to govern his worldly domains. The idea of divine appointment would come to be known as "de jure" laws, or the principles of rightful inheritance. These principles derived their power from the principles of law and the authority of the Church.
The establishment of Feudalism
The new feudal order created a strict and inflexible hierarchy in which the minority of royals, landed aristocrats, and the clergy stood in the far top, a slightly more significant minority of free burghers(wealthy middle class that lived in towns and cities) in the middle, while the masses of serfs were down below in the bottom, chained as involuntary servants for their entire life. This arrangement was by no means fair or moral in the standards of today, but in the past, it was perceived as a natural order designed by no other than God himself.
The decentralised nature of feudalism meant that many fiefs were quick to develop their own cultures, and dialects in the country became broad and varied between small regions. Many forts were built to protect every regional fief and to sustain it and the force that guarded it, agricultural settlements(or villages) developed outside of the fort. This led to many innovations, especially in farming methods and in the design of different fortifications.