  We give a new realization of arbitrary level perfect crystals and arbitrary
level irreducible highest weight crystals of type $D_n^{(1)}$, in the language
of Young walls. The notions of splitting of blocks and slices play crucial
roles in the construction of crystals. The perfect crystals are realized as the
set of equivalence classes of slices. And the irreducible highest weight
crystals are realized as the affine crystals consisting of reduced proper Young
walls which, in turn, are concatenations of slices.
