By reassessing the known primary sources and introducing new archival material from the archives in Hungary, Croatia, and Italy, this thesis revisits several crucial points of the history of Hungarian-Ottoman relations in the era of King Matthias Corvinus (r. 1458-1490) and raises new questions related to the social and political history of the Hungarian-Ottoman frontier. By relying on methods derived from geography and digital history, it challenges the established views about King Matthias’s defensive policies and the role of the so-called ‘southern defensive system’ that relied on royal castles located in the kingdom’s borderlands. The author carefully identifies these castles, sheds light on the chronology of the king’s acquisitions, and highlights the reasoning behind them, showing that the acquisition and the upkeep of at least a third of the ‘frontier castles’ had very little to do with the defence of the kingdom against the Ottomans. Instead, the king tried to check the Venetian influence on his realm(s), fearful of losing primarily Croatia to the Republic. It is further suggested in the thesis that the remainder of the ‘defensive system’ needs to be approached through a geographical lens. After careful analysis, the author shows that rather than an unbroken line of defensive castles that stretched parallel to the boundary between the Hungarian- and Ottoman-held territories, the ‘system’ was a patchwork of clusters of castles grouped around specific parts of the borderland’s terrain, namely valleys and other suitable paths through the Dinaric Alps, where Ottoman troops were able to gain entrance to Hungary. The thesis is further focused on a detailed analysis of the Hungarian-Ottoman negotiations about truces and peace, as well as on questioning whether these two sides ever delineated boundaries. The analysis of all available published and archival sources suggests that a process of bordering never took place and that the territories held by either side remained vaguely defined, signalled only through the locations of frontier castles. It is suggested that the two sides probably never agreed on a long-lasting peace, but rather relied on short-term truces (regardless of the terminology used in the sources). Truces were enforced in 1478, in early 1484, the summer of 1485, and in 1488, and possibly earlier, although there is insufficient evidence to confirm this. The author argues that as the diplomacy relied primarily (if not solely) on the Cyrillic script and the Slavonic language, the only extant copy of an Ottoman ‘ahdname addressed to King Matthias (traditionally dated to 1488 and considered a draft) is a translation of the Slavonic document into Ottoman Turkish. The document was in its entirety or majority first put to paper in 1478, and was thereafter reused in subsequent treaties. According to the extant evidence, the treaties primarily regulated very little beyond the keeping of peace between the two sides, which the Ottoman side granted throughout the late 1470s and the 1480s in exchange for one important concession by the Hungarians, namely the right to pass through Hungarian territory for incursions into the Venetian and Habsburg lands. Lastly, the thesis touches upon several questions related to the administration of the Hungarian frontier, the recruitment of troops, and phenomena of the day-to-day life in the borderlands. It is shown that between 1464 and late 1473, a part of the Hospitaller estates in Hungary were allocated to the king’s captains who governed the frontiers, in order to supply them with additional sources of revenue that was to be directed towards the maintenance of the frontier castles in Bosnia. As the Ottomans relied on various allies, primarily the Vlachs, for the recruitment of units of raiders (the akinji), and Venice on her stradioti, Matthias found ways to use the ever-changing political circumstances in the frontier to attract primarily the local Vlachs and the people of the region of Poljica and employ them for a similar purpose, the petty warfare (Kleinkrieg) that became a regular occurrence in these years. Most of Matthias’s methods for the upkeep of the frontiers or the recruitment of manpower failed, but the king nevertheless constantly searched for new solutions, coming close to establishing a firm structure of defences in the early 1480s. This, the closest that the king had come to the model accepted in current scholarship, also failed not two years later. His methods were never identical to those accepted in historiography (primarily thanks to the work of Ferenc Szakály): built around a stable network of frontier castles, the purposefully recruited troops, and clever management of the kingdom’s resources that secured the means for the upkeep of both the frontier castles and the manpower. In fact, for the majority of his reign, Matthias had very little or no control over the vast regions of the borderlands, he had to navigate through fluctuating political circumstances, the questionable loyalty of his captains, and the local politics over which he had little control in order to primarily preserve his authority in large sections of the borderlands. He similarly had to experiment and frequently come up with new solutions for the defence of Hungary against the Ottoman advance in the remaining sections of the frontier.