When were permanent fortifications usually built?

In the context of military science, permanent fortifications are typically constructed during times of peace or when there is a perceived threat of war.1

Unlike field fortifications, which are rapidly built when contact with an enemy is imminent or ongoing, permanent fortifications involve elaborate structures such as forts and troop shelters designed to provide long-term defensive advantages. Throughout history, these structures have been adapted to counter evolving weaponry, such as the star-shaped fortresses designed by Sébastien de Vauban in the late 17th century to withstand heavy artillery, or the concrete-reinforced forts utilized during the World War I era.123