Osu Castle (also known as Fort Christiansborg or Christiansborg Castle) is a castle located in Osu, Ghana, on the coast of the Gulf of Guinea in Africa.
A substantial fort was built by Denmark-Norway in the 1660s; thereafter, the fort changed ownership between Denmark-Norway, Portugal, the Akwamu, Britain, and finally post-Independence Ghana. Under Denmark–Norway control it was the capital of the Danish Gold Coast, and held and dispatched enslaved people overseas.
The Danish Gold Coast (Danish: Danske Guldkyst or Dansk Guinea) comprised the colonies that Denmark–Norway controlled in Africa as a part of the Gold Coast (roughly present-day southeast Ghana), which is on the Gulf of Guinea. It was colonized by the Dano-Norwegian fleet, first under indirect rule by the Danish West India Company (a chartered company), later as a crown colony of the kingdom of Denmark-Norway. The area under Danish influence was over 10,000 square kilometres.
The five Danish Gold Coast Territorial Settlements and forts of the Kingdom of Denmark were sold to the United Kingdom in 1850. Denmark had wanted to sell these colonies for some time as the expenses required to run the colonies had increased following the abolition of slavery. Although Britain was also struggling with rising costs, it sought to purchase them to reduce French and Belgian influence in the region, as well as to further curtail the slave trade that still operated there. The purchased settlements and forts were later incorporated into the British Gold Coast.