Complex sentence: The fortress was declared a state prison in 1417; this role was expanded first under the English occupiers of the 1420s and 1430s, and then under Louis XI in the 1460s.
Split into simpler sentences: The fortress was declared a state prison in 1417. This role was expanded first under the English occupiers of the 1420s and 1430s, and then under Louis XI in the 1460s.