The castle is a defensive structure with a compact - usually stone or brick - buildings, which is characteristic of medieval Europe, especially in the period of the full (X-XIII century) and late (XIV-XV century) medieval times. The castle served various functions: the seat of royal, ducal or noble power, it could be the "family home" of a single knight, but also a military outpost subordinate to the royal authority. It is assumed that the first "classic" castles in Europe were built in the 9th century, after the fall of the Carolingian state. In many cases - in today's France or Germany - they had the character of the seat of a ruler or a knight and were often only keep, made of stone. These early castles lack many of the later defensive elements, such as towers or an extensive wall system. However, at the turn of the twelfth and thirteenth centuries, there was a significant change and under the influence of the experience of the Crusades, castles evolve, becoming more and more complex. They begin to have one or more rows of walls. In the territory of Poland, castles appeared at the turn of the 12th and 13th centuries, replacing the previous wooden and earth castles, and one of the first such fortresses is the castle in Legnica or the reconstruction of Wawel in Krakow. It is also worth remembering that the Teutonic castles in Pomerania and Livonia were unusual in their own way, because they were built primarily of brick, not stone. A great example of such a castle is, for example, Malbork. The decline of castles dates back to the 15th century, when the popularization of gunpowder made it possible to acquire them relatively quickly.
It is quite commonly assumed that the initiator of the crusades (i.e. the crusades) to the Holy Land was Pope Urban II, who at the Synod in Clermont in 1095 threw the slogan of her liberation from the hands of the infidels. As a result of a combination of various social, economic and religious reasons, this slogan fell on a very fertile ground in Latin Europe and became the cause of the first knight's crusade in the years 1096-1099. As a result of this crusade, not only was Jerusalem conquered, but also in 1100 the Kingdom of Jerusalem was established in the Middle East. Its first king was Baldwin I, although the actual "founder" of the new state was Geoffrey of Bouillon. After the first crusade, there were others, of which we can mention the third (1189-1192) or the fourth (1202-1204), which, however, did not reach the Holy Land, and led to the sacking of Constantinople by would-be crusaders. It is assumed that the end of the crusade movement to the Holy Land is the year 1291, which is the same as the conquest of the last Crusader stronghold in these areas, i.e. Akki. The importance of the crusades in the history of medieval Europe and in the history of its military was enormous. First of all, they led to a significant weakening of the Byzantine Empire, and indirectly facilitated the later expansion of the Ottoman Turks in the 14th-15th centuries. They led to a great increase in the role of Italian cities in the Levantine trade and the development of boatbuilding in the Apennine. The intellectual horizons of contemporary Europeans widened considerably. For the defense of the Holy Land, knightly orders were also established, including the Teutonic Order.