Our main source for information about the Roman kings is Livy Book I-V, the History of Rome. Livy’s dates were 59 BCE – 17CE, so he was writing about events up to 700 years before his time.
Livy was the first professional historian in Rome; historians who preceded him were wealthy people who studied history as a recreational activity. Livy read Thucydides and adopted his methods of relating history as stories about people. Those stories were made interesting to hold the attention of the reader. Livy believed Thucydides’ theory that history was a series of repeatable events displaying human behavior. For example, a tyrant in 500 BCE would act the same as a tyrant in 100 BCE. That idea made it easy to transfer current facts into the past. If the current tyrant stole money from the treasury, then the tyrant, from 400 years before, must have stolen money from the treasury.