This charter tells us the rights and duties of the town and you can now find this letter in the archives of Karlshamn. As already mentioned Karl X Gustav intended to build a port for exports and a naval base. A state shipbuilding yard was however built in 1659 and remained there until 1676, when it was relocated to Kalmar. Plans to protect the city from attacks from the sea were made by Erik Dahlberg. The first fortress was built at Bo n in 1659.
Traces are still to be seen there.
The fortifications at Frisholmen, nowadays Kastellet, started in 1675. The garrison, which at times engaged 400 persons, came to an end in 1864. The town had a couple of times been subjected to the ravages of war. In 1678 the town was paid a visit once again from the land, guided by a traitor called Pickedala-Ola. The town was set on fire and about 20 houses were burnt down to ashes, among them the town hall and the school They managed, however, to drive away the enemy, and the traitor was burnt to death.