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After the fall of Napoleon, many alpine countries developed heavy protections to prevent any new invasion. Thus, Savoy built a series of fortifications in the Maurienne valley in order to protect the major alpine passes, such as the col du Mont-Cenis that was even crossed by, Charlemagne and his father to defeat the Lombarts. The later indeed became very popular after the construction of a paved road ordered by Napoleon Bonaparte. The Barriere de l'Esseillon is a serie of forts with heavy batteries, built on a cliff with a perfect view on the valley, a gorge on one side and steep mountains on the other side.

When did many alpine countries develop heavy protection to prevent new invasion?
After the fall of Napoleon