This traditional account of Roman history, which has come down to us
through Livy, Plutarch, Dionysius of Halicarnassus and others, is that in
Rome's first centuries, it was ruled by a succession of seven kings. The
traditional chronology, as codified by Varro, allots 243 years for their
reigns, an average of almost 35 years, which, since the work of Barthold
Georg Niebuhr, has been generally discounted by modern scholarship.

The Gauls destroyed much of Rome's historical records when they sacked the
city after the Battle of the Allia in 390 BC (Varronian, according to
Polybius the battle occurred in 387/6) and what was left was eventually
lost to time or theft. With no contemporary records of the kingdom
existing, all accounts of the kings must be carefully questioned. The list
of kings is also of dubious historical value, though the last-named kings
may be historical figures. It is believed by some historians (again, this
is disputed) that Rome was under the influence of the Etruscans for about
a century.

-wiki