The effective response of polycrystalline magnesium is dictated by interplay between dislocation slip and deformation twinning. In this work, a constitutive model will be presented incorporating slip systems through crystal plasticity and twinning through a phase-field-type approach. Through stabilized spectral homogenization, pattern formation will be demonstrated in single-slip plasticity when energetic landscapes lose (quasi)convexity. Extended to multiple slip systems and twinning in polycrystals, complex microstructural evolution is shown at the polycrystalline mesoscale. The influence of grain size, structure and texture on the macroscopic response will be quantitatively analysed. Finally, the impact of secondary inclusions and precipitates on mechanical properties will be briefly discussed.