When peace was made at Roskilde in 1658 Bohulän became Swedish. Carl X of Sweden immediately understood the importance of Marstrand´s harbour and ordered the construction of defence works in order to protect it.

The first "fortress" was constructed of earth and wood, but soon the construction of one in stone began. A square tower becmae one of the sides of the fortification. The reamaining three sides, which surrounded a courtyard were constructed in stone. The battlements contained rooms for the garrison. This fortress of fortlet was completed in 1667.

During the 1680s the tower was reconstructed as a circular turret and the construction of another yard was started.

The construction was continued with outer walls, flank defences, batteries, wet and dry trenches and moats until 1860 when the fortress was reported complete.

Already at the beginning of the construction of the fortress there was a problem to find workers. Since a large numbers of hands were needed they started using prisoners. Working on the fortress of Marstrand was introduced as a penalty into the law of Sweden and the number of prisoners consequently increased until it, in the year 1854, reached 232. The same year this work was terminated. Due to the war risk it was not thought advisable to have so many prisoners at one of the most important fortresses of Sweden. Some of the prisoners were transfered to the fortress of Elfsborg and some to the fortress Skansen Kronan in Gothenburg.

In the year 1659, one year after the Treaty of Roskilde, the Norwegians attacked Marstrand. This attack was repelled and it resulted in a greater effort to construct a stronger defence at Marstrand. Peace reigned untill 1677 when the Norwegians and the Danes launched a join attack on Marstrand itself and its fortress. The attack was lead by Ulrik Fredrik Gyldenlöwe, the Danish Governor of Norway, who engaged 5000 soliders in the attack. After three weeks of fighting the fortress had to capitulate. A large part of the turret was then demolished and all the fresh water water was finished.

The Danes ruled the fortress and the island of Marstrand for two years until the Treaty of Lund was signed, when the fortress was returned to Sweden.

The Swededs enlarged the fortress untill 1719 when peder Jansen Vessel Tordenskiold attacked Marstrand with an army of Norwegian and Danish soliders. He succeeded in conquering the city of Marstrand after a rapid attacl. Due to "psycholgical warfare" he managed to make the fortress capitulate a few days later. The fortress remained in Danish hands untill 1720. Since that year it had been Swedish.

The most well-known of all the prisoners is without any doubt the motorius thief Lasse-Maja, alias Lars Lasrsson Molin. He was imprisoned at the fortress in 1813 after having been sentenced to the fortress for the rest of his life. He had to carry stones together with the other prisoner until the commandant found out that Lasse-Maja could cook. He was the moved to the Officers´ mess, where he stayed (exept for one escape in 1823) until 1839 when he was pardonned by Carl XIV Johan of Sweden.

In 1882 the fortress commission had found that the fortress no longer was needed as an active garrison. The navy´s ships had been equiped with steam-engines and as a result the harbour of Marstrand lost its importance. It was now easier to steam out against wind and ice from other ports. On 1 May 1882 the garrison marched out and the fortress closed down.

In 1896 the Swedish Ministry of Defence made the first grants to the restauration of the fortress. This has henceforth continued constantly.

In the year 1907 the navy started a school for ship´s boys at Carlstens Fortress. This military "elementary school" continued until 1937.

Carlsten Fortress is now used for military courses during the winter months. In the summer it si open to tourists when there are guided tours.

Rooms are also left for dinner parties, lunches etc to companies, societies or private people.