Halden had to have defensive amenities, so consequently a fortress was built. This happened back in 1643. This fortress was not strong enough, so King Fredrik III of Denmark ordered that it be strengthened in 1660. This was named the Fredriksten Fortress. The fortress has two ridges and between them, there is a citadel that has a marvellous look on to the city of Halden. Currently, the Fredriksten Fortress is a park where people can walk and explore the hidden places of the fortress.