Complex sentence: This was done so quickly because as a result of a 1615 law mandating one castle per clan, many castles were dismantled and parts of some were used to construct Akashi Castle.
Split into simpler sentences: This was done so quickly because a law had been issued in 1615, declaring that there would only be one castle per clan. As a result, many castles had to be dismantled and parts of some were used to construct Akashi Castle.