Medieval Oslo refers to the urban community of Oslo during the Middle Ages and up until the city fire of 1624, when the town burned to the ground and was not rebuilt on its original site. Instead, the city of Christiania was founded nearby. After this, the original name "Oslo" continued to be used for the original, burned-down area, which was largely converted into farmland. In 1925, the municipality of Christiania changed its name to Oslo, meaning that in modern times, "Oslo" refers to a much larger area than the old medieval town.
The ruins of the medieval town, made of stone and brick, are located in the area now called Gamlebyen ("The Old Town"), roughly bordered by the Hovinbekken stream in the north, the original course of the Alna River to the south and east, and the water features associated with the Medieval Park in the west. The Franciscan monastery (partly overbuilt by Gamlebyen Church and Oslo Hospital) just east of the Alna is also considered part of the Medieval Town, despite being located on the "back side" of the river. Akershus Castle and Fortress (begun in 1297) and Hovedøya with its Cistercian monastery also belong to the Medieval Town.