Complex sentence: Today the donjon is a modern concrete building with air-conditioning and elevators and the Honmaru Palace, which was also lost to the fire in 1945, is currently under construction and should be complete in 2017.
Split into simpler sentences: Today the donjon is a modern concrete building with air-conditioning and elevators. The Honmaru Palace, which was also lost to the fire in 1945, is currently under construction and should be complete in 2017.