  We consider magic-angle twisted bilayer graphene (TBG) at filling $\nu=+3$,
where experiments have observed a robust quantized anomalous Hall effect. This
has been attributed to the formation of a valley- and spin-polarized Chern
insulating ground state that spontaneously breaks time-reversal symmetry, and
is stabilized by a hexagonal boron nitride (hBN) substrate. We identify three
different types of domain wall, and study their properties and energetic
selection mechanisms via theoretical arguments and Hartree-Fock calculations
adapted to deal with inhomogeneous moir\'e systems. We comment on the
implications of these results for transport and scanning probe experiments.
