The reign of Sigismund of Luxembourg and his warfare against the Ottoman Empire is an inexhaustible topic in Croatian historiography. One of the less researched aspects related to this broader theme is his military-defensive system in the middle Danube region. Following the introduction to the topic and the explanation of the methodology, this paper proceeds with a chapter on the existing historiography related to this system. The subs-equent chapters chronologically trace the development of Hungarian-Ottoman conflicts from the emergence of the "Ottoman threat" to the end of Sigismund's reign and life. Along with the chronological division, the paper follows various measures and efforts by which the king sought to halt the advance of Ottoman conquerors. The analysis of various historical sources and existing literature has shown that Sigi-smund attempted to defend his position in three main methods. These methods include the reform of recruitment methods, resulting in a larger number of available soldiers, relations with neighboring vassal states to create a buffer zone, and the formation of a strong fortification system on the Danube between Belgrade and Severin. Historical sources, particularly the let-ters of the Teutonic Order, also reveal the scale of this fortification system as well as their equipment and maintainence. The reasons and events due to which the mentioned system was not sustainable in that form are presented in the end.