Medieval warfare
Organization: The medieval knight was usually a mounted and armoured soldier, often connected with nobility or royalty, although (especially in north-eastern Europe) knights could also come from the lower classes, and could even be enslaved persons. The cost of their armour, horses, and weapons was great; this, among other things, helped gradually transform the knight, at least in western Europe, into a distinct social class separate from other warriors. During the crusades, holy orders of Knights fought in the Holy Land (see Knights Templar, the Hospitallers, etc.).
The light cavalry consisted usually of lighter armed and armoured men, who could have lances, javelins or missile weapons, such as bows or crossbows. In much of the Middle Ages, light cavalry usually consisted of wealthy commoners. Later in the Middle Ages, light cavalry would also include sergeants who were men who had trained as knights but could not afford the costs associated with the title. Light cavalry was used as scouts, skirmishers or outflankers. Many countries developed their styles of light cavalries, such as Hungarian mounted archers, Spanish jinetes, Italian and German mounted crossbowmen and English currours.
The infantry was recruited and trained in a wide variety of manners in different regions of Europe all through the Middle Ages, and probably always formed the most numerous part of a medieval field army. Many infantrymen in prolonged wars would be mercenaries. Most armies contained significant numbers of spearmen, archers and other unmounted soldiers.

Recruiting: In the earliest Middle Ages, it was the obligation of every noble to respond to the call to battle with his equipment, archers, and infantry.  This decentralized system was necessary due to the social order of the time but could lead to motley forces with variable training, equipment and abilities. The more resources the noble had access to, the better his troops would typically be.
Typically the feudal armies consisted of a core of highly skilled knights and their household troops, mercenaries hired for the time of the campaign and feudal levies fulfilling their feudal obligations, who usually were little more than rabble. They could, however, be efficient in disadvantageous terrain. Towns and cities could also field militias.
As central governments grew in power, a return to the citizen and mercenary armies of the classical period also began, as central levies of the peasantry began to be the central recruiting tool. It was estimated that the best infantrymen came from the younger sons of free land-owning yeomen, such as the English archers and Swiss pikemen. England was one of the most centralized states in the Late Middle Ages, and the armies that fought the Hundred Years' War were mostly paid professionals.
In theory, every Englishman had an obligation to serve for forty days. Forty days was not long enough for a campaign, especially one on the continent. Thus the scutage was introduced, whereby most Englishmen paid to escape their service and this money was used to create a permanent army. However, almost all high medieval armies in Europe were composed of a great deal of paid core troops, and there was a large mercenary market in Europe from at least the early 12th century.
As the Middle Ages progressed in Italy, Italian cities began to rely mostly on mercenaries to do their fighting rather than the militias that had dominated the early and high medieval period in this region. These would be groups of career soldiers who would be paid a set rate. Mercenaries tended to be effective soldiers, especially in combination with standing forces, but in Italy, they came to dominate the armies of the city-states. This made them problematic; while at war they were considerably more reliable than a standing army, at peacetime they proved a risk to the state itself like the Praetorian Guard had once been.
Mercenary-on-mercenary warfare in Italy led to relatively bloodless campaigns which relied as much on manoeuvre as on battles, since the condottieri recognized it was more efficient to attack the enemy's ability to wage war rather than his battle forces, discovering the concept of indirect warfare 500 years before Sir Basil Liddell Hart, and attempting to attack the enemy supply lines, his economy and his ability to wage war rather than risking an open battle, and manoeuvre him into a position where risking a battle would have been suicidal. Machiavelli understood this indirect approach as cowardice.

Fortifications: In Europe, breakdowns in centralized power led to the rise of several groups that turned to large-scale pillage as a source of income. Most notably the Vikings, Arabs, Mongols, Huns, Cumans, Tartars, and Magyars raided significantly. As these groups were generally small and needed to move quickly, building fortifications was a good way to provide refuge and protection for the people and the wealth in the region.
These fortifications evolved throughout the Middle Ages, the most important form being the castle, a structure which has become almost synonymous with the Medieval era in the popular eye. The castle served as a protected place for the local elites. Inside a castle they were protected from bands of raiders and could send mounted warriors to drive the enemy from the area, or to disrupt the efforts of larger armies to supply themselves in the region by gaining local superiority over foraging parties that would be impossible against the whole enemy host.
Fortifications were a very important part of warfare because they provided safety to the lord, his family, and his servants. They provided refuge from armies too large to face in open battle. The ability of the heavy cavalry to dominate a battle on an open field was useless against fortifications. Building siege engines was a time-consuming process, and could seldom be effectively done without preparations before the campaign. Many sieges could take months, if not years, to weaken or demoralize the defenders sufficiently. Fortifications were an excellent means of ensuring that the elite could not be easily dislodged from their lands – as Count Baldwin of Hainaut commented in 1184 on seeing enemy troops ravage his lands from the safety of his castle, "they can't take the land with them".

Siege warfare: In the Medieval period besieging armies used a wide variety of siege engines including: scaling ladders; battering rams; siege towers and various types of catapults such as the mangonel, onager, ballista, and trebuchet. Siege techniques also included mining in which tunnels were dug under a section of the wall and then rapidly collapsed to destabilize the wall's foundation.  Another technique was to bore into the enemy walls, however, this was not nearly as effective as other methods due to the thickness of castle walls.

Advances in the prosecution of sieges encouraged the development of a variety of defensive counter-measures. In particular, Medieval fortifications became progressively stronger – for example, the advent of the concentric castle from the period of the Crusades – and more dangerous to attackers – witness the increasing use of machicolations, as well the preparation of hot or incendiary substances. Arrow slits, concealed doors for sallies, and deep water wells were also integral to resisting siege at this time. Designers of castles paid particular attention to defending entrances, protecting gates with drawbridges, portcullises and barbicans. Wet animal skins were often draped over gates to repel fire. Moats and other water defences, whether natural or augmented, were also vital to defenders.
In the Middle Ages, virtually all large cities had city walls – Dubrovnik in Dalmatia is a well-preserved example – and more important cities had citadels, forts or castles. Great effort was expended to ensure a good water supply inside the city in case of siege. In some cases, long tunnels were constructed to carry water into the city. In other cases, such as the Ottoman siege of Shkodra, Venetian engineers had designed and installed cisterns that were fed by rain water channeled by a system of conduits in the walls and buildings. Complex systems of tunnels were used for storage and communications in medieval cities like Tábor in Bohemia. Against these would be matched the mining skills of teams of trained sappers, who were sometimes employed by besieging armies.
Until the invention of gunpowder-based weapons (and the resulting higher-velocity projectiles), the balance of power and logistics favoured the defender. With the invention of gunpowder, the traditional methods of defence became less and less effective against a determined siege.

Relics: The practice of carrying relics into battle is a feature that distinguishes medieval warfare from its predecessors or early modern warfare and possibly inspired by biblical references. The presence of relics was believed to be an important source of supernatural power that served both as a spiritual weapon and a form of defence; the relics of martyrs were considered by Saint John Chrysostom much more powerful than "walls, trenches, weapons and hosts of soldiers"
In Italy, the carroccio or carro della guerra, the "war wagon", was an elaboration of this practice that developed during the 13th century. The carro della guerra of Milan was described in detail in 1288 by Bonvesin de la Riva in his book on the "Marvels of Milan". Wrapped in scarlet cloth and drawn by three yoke of oxen that were caparisoned in white with the red cross of Saint Ambrose, the city's patron, it carried a crucifix so massive it took four men to step it in place, like a ship's mast.

Naval warfare: The waters surrounding Europe can be grouped into two types which affected the design of craft that traveled and therefore the warfare.