Looking at the development of the defensive structures of Gdańsk from the 14th to the 15th century, when Gdańsk, due to its significance in trade, began to emerge as one of the most important components of the economic and political life of Poland, the construction of the fortress in Wisłoujście became of paramount importance. The continuous growth in importance of this point within the overall defense system of Gdańsk can be traced back to its establishment, which dates back to at least the second half of the 14th century. It was during this time that a wooden blockhouse was installed at the mouth of the Vistula River into the Baltic Sea, where a formidable defensive complex still stands today.From then on, the significance of this complex would be shaped by three primary functions. The first function was to shield the city from threats coming from the sea, such as artillery and landing operations by naval fleets operating in the Gdańsk Bay. In this regard, Wisłoujście played a crucial role during the Swedish and Napoleonic wars. Thanks to Wisłoujście, Gdańsk was never caught off guard from the sea.The second function was to secure connections with the outside world. The role of Wisłoujście grew in importance as Gdańsk's economic significance as the main port of the Polish-Lithuanian Commonwealth increased. A complete and effective blockade of the city was impossible without control of the Vistula's mouth.The third function was to create what was known as the "Head of Gdańsk," with a castle in Grabin and fortifications in Kiezmark, forming a broad outer defense perimeter that made it difficult for the enemy to directly attack the city, forcing them to disperse their forces in multiple directions. The utility of the entire system was particularly evident during the war between Gdańsk and King Stefan Batory and during the Swedish wars.Thanks to its military advantages, the fortifications of Wisłoujście were continually expanded and modernized. In 1562, a three-story fortification replaced the old wooden blockhouse, supplemented with additional casemates, blockhouses, and palisades. The next phase of expansion occurred at the turn of the 16th and 17th centuries when a four-bastion fort was built according to the designs of the new Italian school of fortification.Further expansion and modernization of the fortress took place between 1624 and 1626, including the construction of earthworks to protect the masonry walls. This was closely tied to the development of artillery, for which walls without a thick layer of earth became insufficient protection.The fortress played a decisive role for the first time during the war between Gdańsk and Poland in 1577. Its significance repeated several times in Polish history, especially during the Swedish wars and the decline of the Polish state in the 18th century when, in 1734, Gdańsk was the only complex within the country capable of resisting foreign powers. During this period, Wisłoujście served as the base for French landing operations to aid King Stanisław Leszczyński, besieged by the Russians in Gdańsk.In 1806-1807, Gdańsk, along with Wisłoujście, no longer served the Polish cause. Wisłoujście became the launching point for Russian-Prussian attempts to relieve the city besieged by French-Polish forces under Marshal Lefebvre. Wisłoujście and the continuous line of fortifications reaching Holm became the primary focus of fierce battles, with Polish-French attacks and Prussian defenses, as holding or capturing this line would determine the fate of the city dependent on outside aid.On May 15th, a major southward attack by Prussian-Russian forces was repelled by Polish-French forces. Wisłoujście Fortress also played a significant role in 1813 during the siege of the city by Russian forces, when it prevented the besiegers from utilizing their considerable naval advantages.Wisłoujście Fortress is one of those places and objects in Gdańsk that should be visited, although they are not visited as often as they should be. In the 19th century, the fortress served as a prison, and in the interwar period, a sailing club was located there. In 1974, the Wisłoujście Fortress complex became a branch of the Historical Museum of Gdańsk. As one of the two maritime fortresses on the southern coast of the Baltic Sea (alongside the fortress in Baltiysk), and due to its historical value, it has been listed on the World Monuments Watch list of the 100 most endangered monuments in the world. Wisłoujście Fortress is not only a priceless example of fortification architecture but also one of the largest hibernation sites for bats in the Pomeranian Voivodeship.