K. J. Parker.
Sixteen Ways to Defend a Walled City.
Orbit. 2019
This is the story of Orhan, son of Siyyah Doctus Felix Praeclarissimus,
and his history of the Great Siege, written down so that the deeds and sufferings
of great men may never be forgotten.
A siege is approaching, and the city has little time to prepare. The people have no food and no weapons, and the enemy has sworn to slaughter them all.
To save the city will take a miracle, but what it has is Orhan. A colonel of engineers, Orhan has far more experience with bridge-building than battles, is a cheat and a liar and has a serious problem with authority. He is, in other words, perfect for the job.