This was written in the 13th century encyclopedic work De proprietatibus rerum ("On the Properties of Things") Bartholomeus Anglicus (~1203–240). The description of Lithuania as a "land of forests and swamps" given by Bartholomeus proved to be long-standing. This was repeated by various authors up until the 17th century, each author adding new details to it. You can find traces of this idea in the 19th century in the work of Prosper Mérimée. His name was recently brought up by British journalist Anatol Lieven.
Conquerers Frightened by Forests Overrun with Cruel Barbarians
The staying power of this idea is explained by the persistence of stereotypes, the special features of rhetorical culture, the phenomenon of the "long Middle Ages," and the popularity of Bartholomeus' work (after printing was invented, this work was re-issued a number of times). The semantics of Bartholomeus' text is clear, which is the typical depiction of a wild, unbaptized land.
A land, that needed to be conquered, that didn't have cities or castles, because natural barriers took the place of fortifications on it. According to this author, writing in the times of the Crusades, pagans were not the fully-fledged owners of their own wealth; they posed a real or, looking from a modern perspective, imagined threat. However the official position of the Church from the 11th to the 13th century and later (up until the 15th-16th century) was categorical concerning this question.
The "beastliness" and "savageness" of the pagans was noted in various texts of the period. Bartholomeus assigned this latter trait to Lithuanians and Semigallians. He wrote this as the conquering of Prussia and Livonia was underway, while the sources of his information were most likely the priests that visited the colonies that were being formed there. Until this English Franciscan, Bishop Vincent Kadlubek (12th-13th century), who was the chronicler of Casimir II the Just, had described the Pollexiani (a branch of the Yotvingians) and the ancient inhabitants of Prussia. According to the campaign of his ruler the Polish chronicler, writing in the spirit of the Crusades, wrote that the marshy swamps and forests defended the "savage" Pollexiani, while the walls of their cities were "just like those of beasts."
If you don't pay attention to the contemptuous view of Christian authors toward "savages" and the stereotypical nature of their depictions, you can see a kernel of truth in those traits. Lithuanian lands were well-protected by nature, which on a number of occasions helped in war with better armed enemies, especially with the German knight orders in 13th-15th centuries. It was not only the Prussians, Yotvingians and Lithuanians that were able to use the landscape to their advantage. A classical example is the victory of Arminius, who was a chieftain of the Cherusci that defeated the Roman legions of Publius Quinctilius Varus in the Battle of the Teutoburg Forest in the 9th century.
"Knowledge" about Lithuanians in Roman sources and the Order's propaganda
Remembering the methods of medieval authors and how they described Lithuanians, we find that authors from Antiquity used a similar language in their works, writing about the "barbaric" and "savage" Gallic and Germanic tribes that were fighting against the Roman legions. In medieval descriptions, practical experience had a secondary role. What was more important were literary tradition, which followed Antiquity, along with the authority of the Holy Scriptures.
It was typical to use important texts and the "examples" one found in them to support cultures (such as the above-mentioned texts of Julius Caesar and other ancient authors where one can find comprehensive descriptions of the customs and traditions of the "barbaric" Gauls and Germanic tribes). As European states considered themselves as sharing the heritage of the Roman Empire, it was natural to adopt a stance of own vs. barbarians or the Christian vs. non-Christian dichotomy.
Let's remember yet another aspect. There is a poorly understood tie between the swamps, forests and paganism. In the Middle Ages, forests and swamps were thought to be the abode of demons, with pagans the worshippers of evil, the "children of the devil."
Bartholomeus' description was soon enriched by new details. In this case, a weighty contribution was made by Italian humanist Enea Silvio Bartolomeo Piccolomini, who later became Pope Pius II. In his De Europa (1458), we read not only about Lithuania, the land of swamps and forests, but also about the polygamy of Lithuanians, which in the view of the times was a fact. He also mentions "matrimonial assistants," the worship of snakes and forests, as well as other pagan superstitions. A picture of the famous GDL ruler Vytautas is provided that is bathed in hostility, showing him as the "Second Attila." The Teutonic knights, who despised Vytautas and strove to receive support from Europe after the Battle of Žalgiris, threatened Europe with the wish stated by the grand duke that he wanted to hear his horse splash in the Rhine.
Piccolomini mentioned the official baptism of the Lithuanians and provided a story by missionary Jerome of Prague about their hard-fought Christianization, the efforts of which were ultimately for naught. Thus a European reader at the turn of the 16th century thought the situation had not changed all that much, if comparing with the times described by Piccolomini, all the more that the world and its structures were understood as static, while the unconverted pagans, the kind of Lithuanians Piccolomini depicted them (Vytautas banished the zealous missionary John-Jerome on the wish of Lithuanians that became angry with his work), remained "children of the dark." Conversion was understood as a momentary thing (based on the example of Paul the Apostle in the New Testament), and not a process that continues over a long period of time, which is why those who returned to paganism were punished with no mercy. The work of the future pope was influential, and his story about Lithuania ended up in one form or another in the chronicles of Hartmann Schedel (1493) and Marcus Antonius Coccius Sabellicus (1498), the cosmographia of Sebastian Münster, the atlas of Gerardus Mercator and the work of other authors.
In other texts popular in Europe, such as Maciej Miechowita's Treatise on the Two Sarmatias (1517; the author polemicises with Piccolomini and repeats the information of Jan Długosz), or the notes of the journeys to Moscow by Sigismund von Herberstein, ambassador of the Holy Roman Empire, the GDL was described as a sparsely inhabited land with a cold climate and large forests, in which exotic animals multiplied, however a place with few cities, and those that did exist had brick houses. The image taken from the authors of various descriptions and maps of the world spread throughout Europe, the stereotype of a rugged land of forests and swamps was deeply entrenched in the imagination of Europeans for years to come.
Kęstutis Gudmantas, „Miškų ir pelkių kraštas": Keletas pastabų apie Lietuvos įvaizdį XIII–XVII a. raštijoje", in: Inter-studia humanitatis, 2008, Nr. 7, p. 94–113. History