Complex sentence: The forts had a weaker side to the rear to allow them to be recaptured by Belgian forces counter-attacking from the rear and the barracks and support facilities were built on that side, using the rear ditch for light and ventilation of living spaces.
Split into simpler sentences: The forts had a weaker side to the rear, to allow them to be recaptured by Belgian forces counter-attacking from the rear. Barracks and support facilities were built on the rear side, using the rear ditch for light and ventilation of living spaces.