Complex sentence: Her successor, Emperor Paul I of Russia showed no interest in the palace and the massive structure remained unfinished and abandoned for more than 200 years, until it was completed and extensively reworked in 2005-07.
Split into simpler sentences: Her successor, Emperor Paul I of Russia showed no interest in the palace and it remained unfinished. The massive structure remained abandoned for more than 200 years, until it was completed and extensively reworked in 2005-07.