Complex sentence: Instead of excavating new stone, the French-facing side of the now-obsolete "Fort Mutin" was demolished and the resulting material was used to build the two squat, ponderous towers which comprised the new redoubt.
Split into simpler sentences: Instead of excavating new stone, the "French side" of the now-obsolete Fort Mutin was demolished. The resulting material was used to build the two squat, ponderous towers of the new redoubt.