Complex sentence: The architect who drew up the plans for the "little theater" was Pierre-Félix Delarue, who also designed many castles in the region in the second half of the nineteenth century.
Split into simpler sentences: The architect who drew up the plans for the "little theater" was called Pierre-Félix Delarue. He, indeed, was the author of many castles in the region in the second half of the nineteenth century.