  In this Thesis we investigate properties of stability, rigidity and unitarity
of the string landscape in ten and lower dimensions. The dissertation explores
these aspects by intertwining a detailed analysis of string vacua, with and
without supersymmetry, with a bottom-up study driven by unitarity. In
particular, in Chapter 1 the possibility of formulating a necessary and
sufficient condition for the classical stability of non-supersymmetric string
vacua is discussed, emphasising the examples in ten and nine dimensions. In
Chapter 2, new solutions are presented in six dimensions both for BSB and
supersymmetric $T^4/\mathbb{Z}_6$ orientifold vacua, arising from a non-trivial
cancellation of the R-R tadpoles. The consitency of these theories is shown by
checking the cancellation of local anomalies and verifying the unitarity
constraints arising from the introduction of string defects. Finally, Chapter 3
discusses the role played by a new kind of global anomaly, arising from the
inconsistency of effective field theories under topology change. A systematic
analysis of six-dimensional supergravity theories with $\text{SU}(2)$ gauge
group and one tensor multiplet and $\text{U}(1)$ gauge group with no tensor
multiplets is discussed. Novel constraints are found and their cancellation in
string vacua is verified. In addition, we have investigated the
$\text{SO}(16)\times \text{SO}(16)$ heterotic theory compactified on the
$T^4/\mathbb{Z}_6$ orbifold and the Gepner orientifold with no tensor
multiples, showing how such anomalies are cancelled. Even though expected, this
result provides a non-trivial consistency check that is not guaranteed by any
theorem known in the literature.
