Nicknamed the pearl of Haute Provence, Sisteron extends between the Provence and the Alps at the gates of the Baronnies Natural Park and on the edge of the Durance cluse (close valley). The city is nestled at the foot of its citadel, considered by Henry IV as the most powerful stronghold of his kingdom. Sisteron's territory covers 12,400 acres (5,020 ha) facing the Rocher de La Baume, recognizable by the vertical layers of limestone that have made its international reputation and constitute a curiosity for geologists from all over the world.