Complex sentence: In 1980 a profound renovation returned the building to the appearance it had around 1818, a relatively easy task since an enormous number of images from the 18th and 19th centuries depict the Palace in detail.
Split into simpler sentences: In 1980 a major restoration returned the building to the appearance it had around 1818. This task was made easier as an enormous number of images from the 18th and 19th centuries depict the Palace in detail.