  We study different phenomenological aspects of compact, D=4, N=1 Type IIB
orientifolds considered as models for unification of the standard model and
gravity. We discuss the structure of the compactification, string and
unification scales depending on the different possible D-brane configurations.
It is emphasized that in the context of Type I models the $M_W/M_{Planck}$
hierarchy problem is substantially alleviated and may be generated by
geometrical factors. We obtain the effective low-energy supergravity Lagrangian
and derive the form of soft SUSY-breaking terms under the assumption of
dilaton/moduli dominance. We also discuss the role of anomalous U(1)'s and of
twisted moduli in this class of theories. A novel mechanism based on the role
of singularities is suggested to achieve consistency with gauge coupling
unification in low string scale models.
