The fortification work was begun on February 12th 1748 and the work was continued with a rapid pace for approximately ten years, even though there on occasion were shortages of coin or building materials. The rocks were got from Svartholma itself, but bricks were hauled from brick factories in the nearby area. Lime was brought as far as from Gotland. Building rocks were cut by drilling manually and careful splitting without explosives, which would have rendered the rocks useless.
Mainly soldiers and prisoners were used as builders. They were a cheap workforce but skilled craftsmen were also needed to some extent. The worktime was 6 days a week, 12 hours a day. Of the quality of building it can be stated that already in a few years the structures had to be repaired in places.
The central parts of the fortress, the bastions and the curtain walls connecting the bastions began to be finished in the mid 1760ies. The construction work moved to the beaches and the ravelins after this. They were never entirely finished. The shape of the fortress is symmetrical, approximately 120 x 120 metres, and it is made up of four bastions, one in each corner. Three of the bastions were named after officers in charge of the fortification works: Qveckfeldt, von Schantz and Röök. The fourth bastion is called Nordenskiöld after the maker of the first fortification plan of Loviisa.
There are also four curtain walls, named after the cardinal points: northern, southern, eastern and western. The southern curtain wall was for officers. There were the commandant’s apartment and office. The northern curtain wall was a storage and the eastern curtain had living quarters for the soldiers.