Also, though houses face southward in traditional Chinese architecture, houses in Ruyuan broke the rule, which was probably to let the emperors get a view of the lake from all corners of the garden.
Two stones with Emperor Jiaqing's calligraphy show renovation on Ruyuan during the Jiaqing reign (1796-1820).
And, Zhang says that the tiles found at the site have reddish marks on them, indicating that they were burned.
"All the tiles we found had cracks," he says.
"These tiles, only used in imperial institutions were of top quality. The cracks prove that they burned for a long time."
Zhang's team is seeking to establish a date of the fire.
"But as of now I can tell you that the fire was set in the late Qing Dynasty," he says.
"So, it is still premature to connect this fire to the looting of 1860."
However, he says that there are no historical records of Ruyuan after Emperor Xianfeng's reign (1851-61) ended. So, this probably means that the garden was destroyed by then, coinciding with the Anglo-French invasion.
Preliminary investigation of the site began in 2012, but the main project was launched in 2016, and covers an area of 3,800 sq m.
Parts of Ruyuan are still buried today as they are beyond the Yuanmingyuan Ruins Park.
"So, it still depends on the urban planning department if they want to expand our excavation site," says Zhang.
"It will cause trouble if we block the artery－Zhongguancun North Avenue－for archaeology."
Zhang says that the designs of Ruyuan show that there was a pedestrian overpass connecting Ruyuan with nearby gardens, and he says that there are possible ruins beneath the Tsinghua University campus, which is across Zhongguancun North Avenue.
"Yuanmingyuan was much bigger than the ruins we see today," he says.
Though most ongoing archaeological works are not accessible to the public for safety reasons, the Yuanmingyuan site is a popular tourist destination as visitors are allowed to have a look at the archaeologists working.
Yang Yulian from Beijing Administration of Cultural Heritage says: "Archaeology is also a good opportunity to educate the public. And fans of cultural heritage upload pictures on social media and even do live broadcasts."
Yang directs the bureau's education office. She says the Yuanmingyuan model could be replicated at other archaeological sites in Beijing.
As of now some artifacts from Ruyuan are being displayed at the exhibition hall of the Yuanmingyuan Ruins Park, and the event will run through October.
Separately, Zhang says that more archaeological work is to follow at the site.
Archaeological research on the Yuanmingyuan ruins began in 1996, and the excavation of Ruyuan is a part of a five-year project that started in 2015 to figure out the original layout of this complex.
Thanks to the surviving stone relics like fountains and columns, people think that Yuanmingyuan was a Western-style garden, says Chen Hui, a researcher at the administration office of the Yuanmingyuan Ruins Park.
"But Western construction comprises a small part of the architecture in Yuanmingyuan," says Chen.
"It's a pity that Chinese-style architecture was mainly in wood that easily disappeared. So, the excavation can give people a panoramic view of the complex."
Zhang says that it will need a long time to learn about this destroyed wonder in detail.
"I will be satisfied if we can use archaeology to show what 10 percent of Yuanmingyuan was like before I retire," says Zhang.