  We explain the appearance of magic angles and fractional Chern insulators in
twisted K-valley homobilayer transition metal dichalcogenides by mapping their
continuum model to a Landau level problem. Our approach relies on an adiabatic
approximation for the quantum mechanics of valence band holes in a
layer-pseudospin field that is valid for sufficiently small twist angles and on
a lowest Landau level approximation that is valid for sufficiently large twist
angles. It simply explains why the quantum geometry of the lowest moir\'e
miniband is nearly ideal at particular flat-band twist angles, predicts that
topological flat bands occur only when the valley-dependent moir\'e potential
is sufficiently strong compared to the interlayer tunneling amplitude, and
provides a powerful starting point for the study of interactions
