Traveling back to the early Tang Dynasty, he became a marginal prince who was not taken seriously. Nominally he had a noble status, but in fact he was far away from Chang'an and was assigned to join a feudal vassal in a border area.
The fiefdom was barren, the population was sparse, Hu and Han people lived together, bandits were rampant, and they had to face pressures from Turks, Tuyuhun, Western trade routes, and local tyrants. The protagonist knows the general trend of history and masters modern basic technology, so he decides not to be directly involved in the struggle for the throne, but to develop agriculture, salt and iron, textiles, medicine, education, military industry and commerce in the fiefdom.
As the fiefdom became more and more prosperous, he gradually transformed from an "idle prince marginalized by the imperial court" to a powerful frontier king who could influence the fate of the Tang Dynasty.
Detective novel?