Song, 31, together with Li, 30, led a team of 14 designers to produce Visualizing the Great Wall in 2018. Unlike previous illustrated work about Beijing, where the duo grew up and are familiar with, they had to learn about all the sections of the extensive Great Wall from scratch.

"There are more than 4,000 essays and many books and materials about the Great Wall. The huge amount of fragmented information was our biggest challenge when designing the book," Song says.

According to Song, the team discussed dozens of topics pertaining to the Great Wall and decided to use each double-page spread to present one topic with graphics, which was their first challenge.

Song was in charge of designing the chapter about "walls" and tried to find a good way to attract readers.

"We wanted to lead the readers through this chapter as if they are playing a game about how to build the Great Wall-you have to select a location and find the materials to build it," he says.

The next challenge was to design a page that could portray humans and their emotions in different ways through visually-pleasing graphics. For example, there is one topic about generals who had guarded the Great Wall-after several rounds of brainstorming, the designers finally came up with a long-table scene that puts all the generals together, but making different gestures to show their characteristics.

Song's team also chose to tackle issues that have not been thoroughly studied, such as making a page about all the products branded as Changcheng (the Great Wall), and another page delving into the question of whether the Great Wall plays a critical role in wildlife migration.