The fortified tower, which has been extended several times, was built between 1218 and 1220 as the western end of the main street of Bern, a city founded in 1191. With each expansion of the city, which, due to the location of Bern's old town in a bend of the Aare River, could only proceed westward, the tower moved further into the city center, lost its function as a defensive tower, and was subsequently used for other purposes. Before the great city fire of 1405, it served as a prison. Afterward, it was rebuilt as a clock tower – the Zytglogge Tower or simply Zytglogge – using Bernese sandstone. Its clock with striking mechanism displayed the time binding for the entire city. Travel time was measured from the tower. In 1530, the cuboid tower acquired its towering form and a new clock. Decrees and ordinances are posted in the gateway, and since the 18th century, units of measurement for public control have been displayed there. With the late Baroque renovations of 1770/71, it was fully integrated into its surroundings.
Source: Wikipedia.