Complex sentence: The museum was in disarray; prior to the war, construction had begun on a new specially-designed to building to house the collections, although building had ceased during the conflict and it was left abandoned during Cardiff's post-war economic slump.
Split into simpler sentences: The museum was in disarray; prior to the war, construction had begun on a new purpose-built building to house the collections. This had ceased during the conflict and the edifice was left abandoned during Cardiff's post-war economic slump.