Complex sentence: Thus, their big arcades open to the internal part of the towers were closed, and the exterior wall was drilled by several windows (NMS), (5) at the same time that the barbican which crowned the arcades were gradually disappearing.
Split into simpler sentences: Thus, their big arcades open to the internal part of the towers were closed, and the exterior wall was drilled through several windows. At the same time the barbican which crowned the arcades were gradually disappearing.