Frederiksborg Castle (Danish: Frederiksborg Slot) is a palatial complex in Hillerød, Denmark. It was built as a royal residence for King Christian IV of Denmark-Norway in the early 17th century, replacing an older castle acquired by Frederick II and becoming the largest Renaissance residence in Scandinavia. Situated on three islets in the Slotssøen (castle lake), it is adjoined by a large formal garden in the Baroque style.
After a serious fire in 1859, the castle was rebuilt on the basis of old plans and paintings. Thanks to public support and the brewer J. C. Jacobsen, the building and its apartments were fully restored by 1882 when it was reopened to the public as the Danish Museum of National History. Open throughout the year, the museum contains the largest collection of portrait paintings in Denmark. It also provides visitors with an opportunity to visit several of the castle's state rooms including the restored Valdemar Room and Great Hall as well as the Chapel and the Audience Chamber which were both largely spared by the fire and contain sumptuous decorations.
The estate originally known as Hillerødsholm near Hillerød had traditionally belonged to the Gøyes, one of the noble families of Denmark. In the 1520s and 1530s, Mogens Gøye (c.1470–1544), Steward of the Realm, had been instrumental in introducing the Danish Reformation. He lived in a half-timbered building on the most northerly of three adjoining islets on the estate's lake. The property was known as Hillerødsholm (literally islet of Hillerød). After his daughter, Birgitte, married the courtier and naval hero Herluf Trolle in 1544, the couple became its proprietors. In the 1540s, Trolle replaced the old building with a larger manor house.