The Summer Palace is said to be the most well-preserved imperial gardens and the largest of its kind still in existence in China. There’s so much to see and enjoy that most people prefer to stay there at least half a day. Composed mainly of Longevity Hill and Kunming Lake, it owns over 3,000 man-made ancient structures, including pavilions, towers, bridges, corridors, etc. On the grounds of the Palace you will be able to walk through 'The Long Corridor' which is the longest corridor in the world.
https://www.chinahighlights.com/beijing/attraction/summer-palace.htm