A sentry watchtower over the Durance river and surrounding areas, the citadel or fortress of Sisteron controls the narrow gap that separates the high area of Dauphiné and low area of Provence. It is not known when the first fortification was built on this rocky outcrop.

The upper ramparts, the parapets and the dungeon were built in the 13th century at the request of the Counts of Provence. Between 1590 and 1597, following the Wars of Religion, during which Sisteron suffered two sieges, Henri IV commissioned his military engineer Jean Érard to create a modern stronghold. It is probably to him that we owe the essential part of the present fortification. At the end of the following century, following the invasion of the upper Durance by the troops of Victor-Amédée of Savoy, Louis XIV ordered Vauban to design a line of fortifications on the eastern border of the kingdom. Despite an ambitious initial project, the citadel, which is a second line of defense, was only equipped with a single armoury and an upgraded well. In the years 1840-1860, the outer walls were raised, the northern enclosure was redesigned and a spectacular staircase of 350 steps was carved into the rock to reach the Porte du Dauphiné Gate. The citadel, the highest point of the town, dominates the coloured tile roofs of Sisteron perched on its imposing mass of rock. For centuries, the city has lived to the rhythm of garrisons.

The first bell was installed in 1366. It rang the beginning and end of work in the countryside and, until 1841, the curfew at 10pm. Civil and military time were then closely linked. In 1403, the first full clock - which marked 24 hours - refined the division of time. It was replaced in 1417 by a half clock, which was renewed in the 17th century. The clock in the citadel seems to have disappeared in the 19th century, but today the siren in the dungeon still alerts the population in the event of a road accident or forest fire.