Oscarsborg fortress is situated on the shores of the Drøbak Sound, at the narrowest point of the Oslo fjord. It was built to protect Oslo from possible amphibious attack. The oldest buildings present in Oscarsborg were built between 1846 - 1853. The fortress was awarded its first commander in 1853 and a garrison was positioned here in the same year. During a visit to the fortress by King Oscar in 1855 the fortress was named Oscarsborg.