  We calculate the proton lifetime and discuss topological defects in a wide
class of non-supersymmetric (non-SUSY) $SO(10)$ and $E(6)$ Grand Unified
Theories (GUTs), broken via left-right subgroups with one or two intermediate
scales (a total of 9 different scenarios with and without D-parity), including
the important effect of threshold corrections. By performing a goodness of fit
test for unification using the two-loop renormalisation group evolution
equations (RGEs), we find that the inclusion of threshold corrections
significantly affects the proton lifetime, allowing several scenarios, which
would otherwise be excluded, to survive. Indeed we find that the threshold
corrections are a saviour for many non-SUSY GUTs. For each scenario we analyse
the homotopy of the vacuum manifold to estimate the possible emergence of
topological defects.
