Question: The Königsberg fortifications were erected due to the vulnerability during which war of 1563-1721?
Context: The Bell tower in Königsberg (built by the architect Stüler) was a bell tower of the Schlosskirche (Königsberg).  After being largely destroyed in World War II by Allied Forces and then annexed by the Soviet Union thereafter, the city was renamed Kaliningrad, and few traces of the former Königsberg remain today.  As part of Königsberg Castle, the bell tower of the Schlosskirche (Königsberg) was devastated by the 1944 Bombing of Königsberg and 1945 Battle of Königsberg during World War II.  The remnants were demolished in 1968, by which time the city was known as Kaliningrad. The Königsberg State and University Library (German: "Staats- und Universitätsbibliothek Königsberg" ) was a combined state library and academic library in Königsberg, Germany.  One of the most prestigious libraries of the eastern German "Sprachraum", comparable only to the , Königsberg's developed since the 16th century out of several smaller libraries.  It was destroyed in 1944 during World War II. The bombing of Königsberg was a series of attacks made on the city of Königsberg in East Prussia during World War II.  The Soviet Air Force had made several raids on the city since 1941.  Extensive attacks carried by RAF Bomber Command destroyed most of the city's historic quarters in the summer of 1944.  Königsberg was also heavily bombed during the Battle of Königsberg, in the final weeks of the war. The Battle of Zanzibar took place between the German "Kaiserliche Marine" and the British Royal Navy early in the First World War of 1914-1918.  The German cruiser SMS "Königsberg" , while taking on coal in the delta of the Rufiji River (German East Africa), learned that a British cruiser—HMS "Pegasus" , which had been part of the Royal Navy's Cape Squadron sent to counter "Königsberg"—had put in at Zanzibar for repairs.  "Königsberg"′s captain—Commander Max Looff—decided to attack "Pegasus" while she was in port. In its broadest sense, social vulnerability is one dimension of vulnerability to multiple stressors and shocks, including abuse, social exclusion and natural hazards.  Social vulnerability refers to the inability of people, organizations, and societies to withstand adverse impacts from multiple stressors to which they are exposed.  These impacts are due in part to characteristics inherent in social interactions, institutions, and systems of cultural values. The fortifications of the former East Prussian capital Königsberg (now Kaliningrad) consist of numerous defensive walls, forts, bastions and other structures.  They make up the First and the Second Defensive Belt, built in 1626—1634 and 1843—1859, respectively.  The 15 metre-thick First Belt was erected due to Königsberg's vulnerability during the Polish–Swedish wars.  The Second Belt was largely constructed on the place of the first one, which was in a bad condition.  The new belt included twelve bastions, three ravelins, seven spoil banks and two fortresses, surrounded by water moat.  Ten brick gates served as entrances and passages through defensive lines and were equipped with moveable bridges. The Polish–Swedish Wars were a series of wars between the Polish–Lithuanian Commonwealth and Sweden.  Broadly construed, the term refers to a series of wars between 1563 and 1721.  More narrowly, it refers to particular wars between 1600 and 1629.  These are the wars included under the broader use of the term: The Treaty of Königsberg was signed in Königsberg(Królewiec) on May 26, 1390 during the Lithuanian Civil War (1389–1392) between Samogitian nobles and representatives of the Teutonic Knights.  The 31-member or 30-member delegation from seven Samogitian regions (Ariogala, Kaltinėnai, Knituva, Kražiai, Medingėnai, Raseiniai, and Viduklė) arrived to Königsberg around the pentecost.  They promised their loyalty to "their king" Vytautas and guaranteed trade freedom for the Knights in Samogitia.  The Knights gifted the nobles with food and clothes. The Battle of Königsberg, also known as the Königsberg Offensive, was one of the last operations of the East Prussian Offensive during World War II.  In four days of violent urban warfare, Soviet forces of the 1st Baltic Front and the 3rd Belorussian Front captured the city of Königsberg – now Kaliningrad, Russia.  The siege started in late January 1945 when the Soviets initially surrounded the city.  There was heavy fighting for the overland connection between Königsberg and the port of Pillau, but by March 1945 Königsberg was hundreds of kilometres behind the main front line.  The battle finished when the German garrison surrendered to the Soviets on 9 April after a three-day assault made their position untenable. The Krepost Sveaborg was an Imperial Russian system of land and coastal fortifications constructed around Helsinki during the First World War.  The purpose of the fortress was to provide a secure naval base for the Russian Baltic fleet and to protect Helsinki and block routes to Saint Petersburg from a possible German invasion.  Krepost Sveaborg was part of Peter the Great's Naval Fortress, a coastal fortification system protecting access to Saint Petersburg by sea.  The central part of Krepost Sveaborg was the old fortress of Suomenlinna where the fortress headquarters were located.  Due to technological advances in artillery the old fortress was no longer capable of providing a sufficient protection, and a new main defensive line was built well beyond the old fortress boundaries.  New coastal artillery guns built on outlying islands protected Krepost Sveaborg from the sea, while fortified lines constructed around Helsinki were intended to stop any attacks on land.  The primary coastal guns were 10 in model 1891 guns and 6 in model 1892 Canet guns. Older 11 in model 1877 guns were also used.  In summer 1917 the fortress had two hundred coastal or anti-landing guns, of which 24 were 10-inch guns in six batteries, 16 were 6-inch Canet guns in four batteries and twelve were 11-inch guns in three batteries.  The artillery used in land fortifications included older coastal guns, old fixed carriage guns and newer light field guns.  In March 1917, Krepost Sveaborg had a total of 463 guns, although many of them were obsolescent.  Krepost Sveaborg was still partly incomplete in 1917 when the February Revolution halted most of the construction work.  Some further construction work was carried out during the remaining year, but all work halted during the October Revolution.  Following the Finnish Declaration of Independence, parts of the land fortifications were used in the Finnish Civil War.  The coastal fortifications were later taken over by Finland to protect Helsinki, while the land fortifications were mostly abandoned and disarmed.
Answer: The Polish–Swedish Wars