I'm reading Livy and he talks about how after Tarquinius Superbus the Roman's hated monarchy. There are numerous examples of fear of a particular figure becoming a tyrant and the loss of "libertas".
Liberty in the Roman context doesn't seem to line up with the modern western view of liberty, so what did it mean to the Romans? How did the presence of a monarch impede liberty? I don't get the impression of a large shift towards greater freedom with the advent of the republic.
The standard go-to for this question is still Wirszubski, Libertas as a political idea at Rome (1950). If you PM me, I can help you get access to this. He lays everything out in the introduction.
Roughly, though, he says that:
libertas is not innate, but is instead the sum of civic rights and prerogatives granted by or through Roman law
it is defined by restraint, both internally and from external sources; it is bounded by disciplina (compared to licentia), by Roman custom (the mos maiorum), and by precedent (in some way).
Roman libertas is not about the individual, but rather the person as they interfaced with the community; this alone makes the concept quite alien to modern conceptions, I think. Wirzubski says that "...libertas at Rome was not the watchword of the individual who tried to assert his own personality against the overriding authority of society" (p. 8).
It is a really good study and the introduction on this topic is def. worth a read (rather than my poor summary)!