We really don’t have a lot of reliable historical information about the Kingdom of Rome. Some of the most critical scholars have claimed that Rome was never a monarchy at all. More cautious historians look at ‘monarchic institutions’ instead of the actual kings. Generally, I think most believe that there was a regal period, but information is very shaky about it.

The traditional list of seven kings is almost universally claimed to be legendary as is. The length of their reigns seem suspiciously long compared to normal human life expectancy, for instance, and many parts of their stories have proven folkstory elements inserted. However, some scholars think that educated guesses can be made from the traditional narrative. Be warned that practically all of these ideas have been challenged at one time or another, though.

For example, apparently the title of king was not hereditary. Based upon the tradition the monarchy was elective, but one in which there were connections between kings and their successors. This could involve marriage, like how Servius Tullius was the son-in-law of Tarquinius Priscus. A king could also give a chosen successor a position of responsibility. This is interestingly similar to the much-later situation with the elective (ed. I meant adoptive!) emperors, you might notice.

Another fascinating feature is that many kings were outsiders according to the tradition. Early Rome was a very open society, but this seemed to also include their kings. Numa was a Sabine and Tarquinius Priscus was a Greek-Etruscan, for instance.

Apparently the kings were also not patricians (although Dionysius of Halicarnassus claims they became patricians upon accession). This suggests that the king of Rome had to be an outsider and that members of the native patrician aristocracy were not eligible. This could have been an innovation they agreed upon so no one clan would gain ascendency over the other. The procedures for choosing a king were extremely complex and involved heavy religious elements and all sections of society (‘with the authorisation of the Fathers, by command of the People’).

Besides the written traditions we can learn a bit from archaeology and ‘kingly’ institutions which survived into a later period which gives more reliable historical texts.

In 1899 a stone was unearthed with an inscription in very early Latin which refers to a king. However, this might not refer to the actual king, but to the rex sacrorum – an office which carried out kingly religious duties into the republic. Some have wondered if the office of rex sacrorum meant that the kings gradually lost real power and became a figurehead with only a ceremonial and religious role.

There are other ‘institutional fossils’ which survived into later periods which hint at kingly duties or festivals. When the king disappeared these duties needed to continue with other officials, some of whom used pseudo-kingly titles. This leads us to suspect that the office of king had very important religious duties, and did not only include secular governance powers.

An excellent source is T.J. Cornell’s The Beginnings of Rome. A good counterpoint which challenges Cornell on many issues is Gary Forsythe’s A Critical History of Early Rome.

It's worth mentioning that the Romans didn't know a lot about the Kingdom of Rome. The early Kings were semi-mythological even in the time of the Republic.