So! A word about the use of Thăng Long as the capital.
The early history of Vietnam is complicated, and is bound up in Chinese history as well. During the fall of the Qin dynasty, a Qin loyalist set up a semi-independent kingdom of "Nan Yue" in the area of southern China and northern Vietnam. The name means "southern Yue", which then begs the question... who are the Yue? The word - rendered "Việt" in Vietnamese - meant something like "peoples beyond the boundary". i.e. the people outside of the Chinese empire. Combined with "Nam" or "Nan" - "south", and the meaning is obvious. So who were these people? The name indicates a multi-ethnic group of people, including speakers of Vietnamese, Tai, Lac, Cantonese, Hokkien, Teochieu, and other languages. "Yue language" is still a term used to describe the Chinese dialect in Guangdong. Additionally, the territory that Nanyue occupied was mostly in present-day China. Nanyue was eventually conquered by the Han Dynasty, and a process of Sinification started.
It was against this process that rebels in many places emerged, but the Trưng sisters and Triệu were (obviously) both in the area of northern Vietnam and have thus become Vietnamese folk heroes.
So here's the question. What's Triệu's capital? Her military camp was hardly a city, and Hanoi didn't exist at the time. So what to go for?
We originally thought to go for Hanoi, just to be recognizable, but it seemed to put a heavy focus on more modern-day Vietnam when we wanted an older feel. We especially did not want Vietnam to be reduced to an "American experience in Vietnam" trope. With that in mind, Thăng Long was just too attractive of a name ("rising dragon") and a historical period. So there's a slight anachronism here - we could not have an appropriate capital for Triệu (as she was not the ruler of a city), nor could we use Nanyue city lists (as most of those would overlap with China), and, besides, during Triệu's life the entire country was occupied, so we went for the next available option - city names and locations as of the first real moment of an independent Vietnamese kingdom (and, further down the city list, later cities extending to the south).
Finally, diacritics were important. The word "thanh," for instance, is not the same as the word "thành". While there are fully Romanized spelling conventions, we thought to keep the Vietnamese spelling to preserve the feel of each city.