Complex sentence: In and around Fukuoka Castle, several Buddhist temples were placed at strategic locations, probably because temple buildings could be used as barracks, temple courtyards as assembling places, and tombstones from temple cemeteries as material to build stone walls.
Split into simpler sentences: In and around Fukuoka Castle, several Buddhist temples were placed at strategic points. This was probably because temple buildings could be used as barracks, temple courtyards as assembling places, and tomb stones from temple cemeteries could be used to build stone walls.