Short answers, pulled from the story.
Kronborg is a Renaissance castle in Helsingør, Denmark, famous as the setting of William Shakespeare's tragedy Hamlet, Prince of Denmark, where it is called Elsinore. It was inscribed on the UNESCO World Heritage List in 2000 and is considered one of the most important Renaissance castles in Northern Europe.
The original fortress on the site, called Krogen, was built in the early fifteenth century by King Eric VII of Pomerania. The castle was radically rebuilt as the Renaissance stronghold known as Kronborg between 1574 and 1585 under King Frederick II, with the Flemish architects Hans Hendrik van Paesschen and Anthonis van Obbergen overseeing the work.
Hamlet was first performed at Kronborg in 1816 by soldiers from the castle garrison. Later productions featured Laurence Olivier, John Gielgud, Christopher Plummer, Derek Jacobi, David Tennant, and in 2009 Jude Law. In 2017, Cyron Melville played Hamlet in a production directed by Lars Romann Engel with music by Mike Sheridan.
A fire caused by two careless workmen broke out on the night of the 24th to the 25th of September 1629, destroying much of the castle. Only the chapel survived intact. King Christian IV had the castle rebuilt under architect Hans van Steenwinckel the Younger, with exterior reconstruction completed by 1639, though the interior never fully recovered its earlier splendour.
Kronborg sits at the narrowest point of the Øresund, where the strait between Denmark and Sweden is only four kilometres wide, controlling one of the few exits from the Baltic Sea. King Eric VII used the position to collect Sound Dues from every ship passing through, with the castle's guns ensuring compliance from all merchant and naval traffic.
Holger Danske, also known as Ogier the Dane, is a legendary Danish king linked to Arthurian myth who was taken to Avalon by the enchantress Morgan le Fay. According to legend, he sleeps at Kronborg Castle with his beard grown along the ground, ready to wake and defend Denmark if the country is ever in danger. A statue of the sleeping Holger Danske has been placed in the castle's casemates.