Osu Castle, UNESCO World Heritage castle in Osu district, Accra, Ghana

Osu Castle is a whitewashed complex behind a perimeter wall in Osu, Accra, its upper storeys pierced by rows of small dark windows and topped by a shallow red roof between two thin masts. A tall white cylindrical water tower with a flat-topped tank stands to the right. The older building beside the road has two rows of arched windows with closed dark shutters, a flat-roofed section ending in a small square tower with a domed cap, and peeling paint above the lower windows.

The Portuguese first occupied the site in 1550; the Swedish trader Heinrich Carloff obtained permission from the King of Accra in 1652 to build a small fortified lodge, and Denmark–Norway took control soon after. The fort was named Christiansborg after King Christian V and from 1685 served as the capital of the Danish Gold Coast. Portugal held it briefly as Fort São Francisco Xavier, and the Akwamu occupied it in 1693 before selling it back in 1694 while keeping the keys. Britain bought all Danish Gold Coast possessions in 1850, and in 1902 the castle became the seat of government.

The name Christiansborg honours King Christian V of Denmark, and the Akwamu kept the castle keys after selling it back in 1694; the keys remain with the Akwamu today. A room was added for Queen Elizabeth II's 1961 visit, a year after Ghana became a republic.

Osu Castle stands on the Gulf of Guinea coast in Osu, central Accra, near Fort Crèvecœur and Fort James. The castle ceased to be the seat of government in 2013, when functions moved to Jubilee House, so access is arranged rather than open. The whitewashed walls and shuttered arched windows are best seen from the road outside.

When the Akwamu seized the fort in 1693, their leader Assameni held it for a year, trading with merchants of many nations, then sold it back to Denmark-Norway in 1694 for 50 marks of gold. He kept the keys, and they remain in Akwamu possession today.