At the Beginning, My Daughter Is a Slave in the Mines, and I Become the King by Making Weapons.

Shen Zheng, a researcher at the Modern Weapons Research Laboratory, accidentally traveled to the ancient Daughter's Kingdom and became a mining slave. Every time he took in a woman, the Daughter's Kingdom's strength jumped to a higher level in a competition with foreign enemies - from crossbows to artillery, from a weak country to a hegemon.