The paper discusses the existence of one-way edge modes in gyromagnetic photonic crystals, which are analogous to quantum Hall edge states. These modes can appear in more general settings than previously thought, specifically in photonic crystals with gyrotropic constituents without the need for Dirac points in the band structure or gyroelectric materials. The authors derive an analytical mapping between the electromagnetic modes in these photonic crystals and the wavefunctions of a nonrelativistic electron in a quantum Hall system, linking photonic one-way modes to QH edge states via the Hatsugai condition. They design an experimentally feasible one-way waveguide using a 2D MO square-lattice Yttrium-Iron-Garnet (YIG) photonic crystal operating at microwave frequencies. The waveguide consists of an interface between an MO photonic crystal and a gapped material, such as a regular 2D photonic crystal. With realistic material parameters, the one-way modes are laterally confined to a few lattice constants and occupy a broad ( approximately 10%) band gap with negligible material loss. Numerical simulations show that these edge modes are immune to backscattering, even from large defects like perfect conductors. The results suggest potential applications in slow-light structures, microwave isolators, and high-Q channel add/drop filters.The paper discusses the existence of one-way edge modes in gyromagnetic photonic crystals, which are analogous to quantum Hall edge states. These modes can appear in more general settings than previously thought, specifically in photonic crystals with gyrotropic constituents without the need for Dirac points in the band structure or gyroelectric materials. The authors derive an analytical mapping between the electromagnetic modes in these photonic crystals and the wavefunctions of a nonrelativistic electron in a quantum Hall system, linking photonic one-way modes to QH edge states via the Hatsugai condition. They design an experimentally feasible one-way waveguide using a 2D MO square-lattice Yttrium-Iron-Garnet (YIG) photonic crystal operating at microwave frequencies. The waveguide consists of an interface between an MO photonic crystal and a gapped material, such as a regular 2D photonic crystal. With realistic material parameters, the one-way modes are laterally confined to a few lattice constants and occupy a broad ( approximately 10%) band gap with negligible material loss. Numerical simulations show that these edge modes are immune to backscattering, even from large defects like perfect conductors. The results suggest potential applications in slow-light structures, microwave isolators, and high-Q channel add/drop filters.