Complex sentence: Work on this expansion started quickly but was soon interrupted, leaving the foundations buried under a platform on which the royal stables were later built, these were demolished in the 20th century and replaced by the Sabatini Gardens.
Split into simpler sentences: Work on this expansion started quickly but was soon interrupted, leaving the foundations buried under a platform on which the royal stables were later built. These were demolished in the 20th century and replaced by the Sabatini Gardens.