Japan

Literally: "capital of the East". Note that the Chinese term 东京 (dong1 jing1) is highly ambiguous in historical contexts, as many places throughout history have been known as "the capital of the East".Of of those other places is the Kaifeng, which is the Capital in Water Margin and in real life during the Song dynasty. The novel is also likely highly popular in Japan apparently as well BTW, e.g. it merited a well funded adaptation: The Water Margin.

As quoted on Ghost Dog: The Way of the Samurai (1999) at 00:23:01:

Among the maxims on Lord Naoshige's wall there was this one: "Matters of great concern should be treated lightly." Master Ittei commented, "Matters of small concern should be treated seriously."

TODO find exact page/section.