Complex sentence: Prince Leopold's wedding cake was created in separate layers with very dense icing, when the icing hardened the tiers were then stacked, this method had never been used before, it was a groundbreaking innovation for wedding cakes at the time.
Split into simpler sentences: Prince Leopold's wedding cake was created in separate layers with very dense icing. When the icing hardened the tiers were then stacked; this method had never been used before, and it was a groundbreaking innovation for wedding cakes at the time.