A French cannon founder, Peyrembert, received permission to build a cannon factory here. Having gotten into difficulties, King Frederik V requested that his chancellery advisor, Johan Frederik Classen, take over the operation of the foundry. Under Classen's management the town blossomed, and there came a gunpowder mill, as well as light industry and handicrafts to the town. This resulted in Classen's being appointed Major General. On August 25, 1756 the king issued a document permitting Classen to call the town "Friederichswerk".
Classen was allowed to build a new foundry, Gjethuset, and it was constructed between 1761 and 1767. It has been used as a cannon foundry until 1928, and has been used for heavy industry until 1976. The building has been restored and reopened on June 12, 1996. It is now used as an art and culture center with theatre, music and art exhibitions. The original architect was either Laurids de Thurah or Nikolai Eigtved. The word Gjethus comes from Low German Gethus and means "foundry house".
Classen's home in the neoclassical style, Arresødal, is still standing. It is in private ownership, but can be viewed from the outside.
There are a number of churches from the Middle Ages in the villages near Frederiksværk. Vinderød Church, in the town of Frederiksværk, houses the sarcophagus of General Classen and has a view towards the Arresø.
Frederiksværk Town Museum 
The Town Museum is established in the old Arsenal, which was used as a storehouse for the cannons cast in Gjethuset. Displays in the museum concern weapon fabrication, gunpowder production and copper rolling during the years 1750 to 1900. There is also a restored grocery store from 1900.
The Gunpowder Mill Museum 
The small open air museum, part of the Industrial Museum Frederiks Værk, contains one of the world's best preserved powder mills with buildings and working machines from 1800. There is also a working watermill on the premises.
Castle ruins 
The ruins of Dronningholm Castle lie near the Arresø. It was started by Valdemar the Great, and continued by his son Valdemar the Victorious. The castle was one of the largest in the country. but burned down in 1525. According to legend the castle was given as a gift to Queen Dagmar.
The ruins of Asserbo Castle are also open to the public for viewing and touring. Asserbo Castle, which lies in the woods of Tisvilde Hegn, was founded by Bishop Absalon in the 12th century and was used as a monastery for monks of the Cistercian Order, originally invited from France. The King took claim of the castle ca. 1560, and then by drifting sands. The castle was reclaimed from the sands, first by King Frederik VII in 1849, and then by the National Museum of Denmark excavations in 1972.
GeoArt - Art gallery & Crystalmuseum 
GeoArt is located at the northern top of lake Arre, just between Frederiksværk and Helsinge. Take the bus from either of the cities or a nice walk in the forest from the Castle ruins (2 km). Or if you feel more like driving all the way just park right outside on one of the 40 parking lots.
GeoArt has an art gallery with changing exhibitions and a permanent exhibition of gems and minerals. For example 225 mill. year old petrified wood from Arizona, Amethyst from Brazil, Flourith from China etc. There's a cafe at the 1st. floor and the shop downstairs sell gemstones from all over the world as well as jewellery and handicrafts/furniture from Thailand, Indonesia and Laos. Well worth a visit if you are in the area, check the GeoArt website for opening time (åbningstider) details.
Frederiksværk municipality 
Until January 1, 2007, Frederiksværk was also a municipality (Danish, kommune) in Frederiksborg County on the island of Zealand (Sjælland) in eastern Denmark, located between Lake Arre (Arresø) and Roskilde Fjord. The municipality covered an area of 90 km², and had a total population of 20,340 (2005). Its last mayor was Helge Friis, a member of the Social Democrats (Socialdemokraterne) political party.
Frederiksværk municipality ceased to exist due to Kommunalreformen ("The Municipality Reform" of 2007). It was merged with existing Hundested municipality to form the new Frederiksværk-Hundested municipality. The name was changed to Halsnæs Municipality on January 1, 2008. This created a municipality with an area of 120 km² and a total population of ca. 30,253.
Municipal information 
- Municipality's official website (Danish)
- Municipality's tourist bureau.
- Municipal statistics: NetBorger Kommunefakta, delivered from KMD aka Kommunedata (Municipal Data)
- Municipal mergers and neighbors: Eniro new municipalities map