I want to know the answer to the following question:
When did many alpine countries develop heavy protection to prevent new invasion?
All the background I'm provided with is that "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.". Is that enough to answer the question?