  Surfaces of multilayer semiconductors typically have regions of atomically
flat terraces separated by atom-high steps. Here we investigate the properties
of the low-energy states appearing at the surface atomic steps in \snte. We
identify the important approximate symmetries and use them to construct
relevant topological invariants. We calculate the dependence of mirror- and
spin-resolved Chern numbers on the number of layers and show that the step
states appear when these invariants are different on the two sides of the step.
Moreover, we find that a particle-hole symmetry can protect one-dimensional
Weyl points at the steps. Since the local density of states is large at the
step the system is susceptible to different types of instabilities, and we
consider an easy-axis magnetization as one realistic possibility. We show that
magnetic domain walls support low-energy bound states because the regions with
opposite magnetization are topologically distinct in the presence of
non-symmorphic chiral and mirror symmetries, providing a possible explanation
for the zero-bias conductance peak observed in the recent experiment [Mazur et
al., Phys. Rev. B 100, 041408(R) (2019)]
