Complex sentence: In 1638 a new addition to the Kloveniersdoelen was completed that was very modern for its for its time--rectangular and in the Classicist style that was very fashionable at that time.
Split into simpler sentences: In 1638 a new addition to the Kloveniersdoelen was completed. It was very modern for its for its time--rectangular and in the Classicist style that was very fashionable at that time.