The building was mentioned for the first time in 1453. Sławno had three gates. Two of them - the Koszalin and Słupsk ones survived to this day. The names of nearby streets were derived from the names of the gates, so Koszalin Street (now - Jedności Narodowej Street) ran near the Koszalin Gate . An important Szczecin - Gdańsk trade route led through both gates. On the map of Pomerania by E. Lubinus dated 1618, on the edges of which panoramas of selected Pomeranian cities were placed, there is also Sławno. You can see the original appearance of the gate on this map. The gate is one of the few relics of the old fortifications of the city.
It is a freestanding building, built in the Gothic style of solid ceramic brick, in Monk bond, on a square-like plan. Field stones form the foundations. The gate is a four-storey one. There is a crossing with a ogival arch in the bottom storey. The gate is now covered with a hipped roof. It was originally a gabled roof. The northern and southern elevation is decorated with six elongated, slender blinds with ogival arches at the ends. Each blind is divided into two ogival windows with circular rosettes above them. The eastern elevation has three blinds, and the western one - two blinds. There used to be embrasures in the elevations of the gates, but turned into window openings. Part of the original embrasures was preserved in the fourth storey of the gates, but they are visible only from the inside. The walls of the gate openings are visible putlog holes that were used to mount the beams of the scaffolding whenever renovation works were carried out. In the eighteenth century, the walls and town gates were gradually losing their importance. Deprived of a military role, the town walls have had a changeable status throughout history.
The Koszalin Gate suffered the same fate. In 1738, the local garrison became the owner of the gate, which adopted rooms for workshops after renovation works. In 1913, the gate passed into the hands of youth organisations. After World War II, fire-fighters used it for drying fire hoses. In the 1980s, the gate was taken over by the House of Culture in Sławno, then, successively, by Naczelna Organizacja Techniczna [Polish Engineering Association], the House of Culture for the second time; and, finally, the Municipality of Sławno. Currently, the gate is not used.
The further fate of the historic building remains a mystery. Undoubtedly, the Koszalin Gate, like the Słupsk Gate have already rooted in the landscape of the city so deeply that modern residents will want to keep these 'witnesses of history' in good condition for future generations.