Warning - there are no direct drama spoilers but this drama has to follow history so if you read this you may be "spoiled" over which side wins.
This drama uses era names a lot:
Hongwu era was the founding Ming emperor Zhu Yuankuang's long reign of ~30 years, outliving his son and heir, the crown prince Zhu Biao, posthumously named Xiaokang emperor by his son, Zhu Yunwen. Zhu Biao was the crown prince during the Hongwu era that often comes up in this drama.
Jianwen era (current era) - This is Zhu Yunwen's short reign which lasted under 5 years. Zhu Yunwen was Zhu Biao's son and Hongwu's grandson and chosen heir after Zhu Baio died. This drama unfolds in the third year of the Jianwen era. The end of this second Ming emperor's reign is steeped in mystery that leaves ample room for historical speculation.
Yongle era - Zhu Yunwen was succeeded by Zhu Di, the Yongle emperor. Zhu Di was one of the most controversial emperors in Chinese history - as glorious and successful as he was unbelievably cruel. He was most recently portrayed by Feng Shaofeng in The Imperial Age (2022). Zhu Di is Prince Yan.
Prince Yan was Hongwu's fourth and some say, favorite son. He was the Jianwen emperor's most powerful uncle prince who ferociously guarded the Northern border from the Mongols since the Jianwen emperor was still in diapers. When the drama opens the country is already at civil war, because Prince Yan has rebelled against the emperor. This big event is just a whispered casual conversation between Lu Yulin and the Minister of War (ep 1), which makes no sense. It should just be set as the context upfront. The 夜不收/Yebushou or night scouts were Prince Yan's elite border spies so they work for him. The 锦衣卫/Jinyiwei or the brocade guards of course works for the emperor as his secret police. So it is spies vs spies.
Prince Yan rebelled for many reasons. Years ago, Hongwu Emperor did not name him the crown prince even though he was more capable because he made a rule that the crown prince must be the son of the empress or the legitimate wife. So he chose Zhu Biao crown prince because Zhu Biao's mother was Hongwu's empress. When Zhu Biao died (was murdered?) Hongwu chose his grandson Zhu Yunwen to succeed him but there were many rumors he also seriously considered Prince Yan. This may have been hard for Prince Yan to accept because Zhu Yunwen's mother, like Prince Yan's mother, was only a concubine. But the straw that broke the camel's back was when the young and easily influenced Jianwen Emperor listened to court officials and started to try to cull the power of his uncles (Hongwu had 26 sons) and killed/arrested some of them. Prince Yan not only tirelessly defended the border against the Mongols, he was the young emperor's most powerful uncle. He was not about to do nothing and let his nephew emperor come for him.
The background is a battle for the throne. There is no right or wrong side. Many of the officials in the capital are "hedging their bets" by playing both sides of the conflict so it is very difficult and very easy to figure out everyone's true loyalties. Most of them will support the winner whoever it turns out to be. But there will be one or two that are entirely loyal to one side.
Locations
Yingtian - This southeastern city was the capital of the Ming Dynasty during the Hongwu and Jianwen reigns. Today it is known as Nanjing in Jiangsu Province.
Jinan - This is where Wang Kegong returned from and it is to the north of Yingtian. Prince Yan fought hard for control of this city, capital of Shandong Province.
Beiping - This is the northern most of the three cities and was Prince Yan's feudal seat and where he waged war on his nephew from. Today it is Beijing. The capital and seat of the empire eventually moved to Beiping.