Complex sentence: Beforehand, wooden scaffolding, overlaid with curtains, had hidden the fountain, though probably not the obelisk, which would have given people an idea that something was being built, but the precise details were unknown.
Split into simpler sentences: Beforehand, wooden scaffolding, overlaid with curtains, had hidden the fountain, though probably not the obelisk. People would obviously have had an idea of something being built, but the precise details were unknown.