Due to the construction of the Noordhollandsch Kanaal, Den Helder also became the access to the port of Amsterdam.

In 1875 the minister of war Enderlein came with a specific plan to build a fortress on the shoal Harssens, on the east bank of the Nieuwediep.

[5] In 1879 Minister of War Den Beer Poortugael took action by amending the fortifications budget law for 1879.

[7] There were some doubts about the plan, but many also understood that in view of the higher cost of alternatives, Fort Harssens was a realistic and effective option.

[9] On 16 August 1879, the construction of the flood resistant glacis of the fortress was awarded to contractor J.J. Bakker from Lent for 143,900 guilders.

The first was to buy the first two 30.5 cm MRK L/25 guns, a test carriage and 100 shells for 221,000 guilders at Krupp.

The lighter carried a wagon on which the plates would be transloaded, on the other side, the vessel align its rail with that on the shore, by using a hydraulic system.

[18] On resumption of the tests, an elevation cylinder filled with glycerol broke and put the second carriage out of action.

[21] The plans for Fort Harssens were based on the idea that by itself, the fortress would have to be able to fight a strong enemy fleet and resist it.

[22] Already in 1879, it was known that the Minister of War wanted the fortress to get two armored cupolas with two 30.5 cm MRK L/25 guns each.

That year, the minister explained that at a distance of up to 2,400 m, the 30.5 cm Krupp guns were able to penetrate the armor of the heaviest battleship that could approach Den Helder.

[23] Some members of parliament wanted to economize by using lighter guns and compensating for this by using heavier armor.

Some members of parliament doubted this, because the minister did think lighter guns good enough for the fortress at IJmuiden.

At IJmuiden, enemy ships could only close in by steaming up the outer parts of the canal, making them an easy target.

[24] The Dutch government had made some tests of the cupola's armor against its new Krupp 25 cm L/25 gun part of its contract with Gruson.

They were attended by the whole German artillery test commission, Henri Alexis Brialmont, the Japanese and Chinese ambassadors etc.

[25] With the fourth hit from the gun, the armor plate broke and parts of it fell inside, thus failing the test.

Positive notes were that the shot had not penetrated the armor, and that the Gruson chilled iron was of a uniform quality.

The net result of the test was that the front plates were not accepted, and Gruson was told to improve these.

[36] In 1886, the Dutch Ministry of War formed a new organization to man the three armored fortresses that were under construction: Harssens, IJmuiden, and Hoek van Holland.

On the contrary, at Terschelling the loud shots led to steamboats seeking for a ship in distress.

Already before World War II, Dutch ships like the flagship De Ruyter were too long for the old harbor.

In relation with the plans for a new office and maritime control center, one of the dry moats was excavated in 2009.

[46] On 12 July 2011 the new Haven Coördinatie Centrum Den Helder was officially opened by Mrs. Elisabeth Post, deputy of the King's commissioner in Noord-Holland.