We perform a comprehensive analysis of uniform-velocity bilayer spacetime crystals, combining concepts of conventional photonic crystals and special relativity. Given that a spacetime crystal consists of a sequence of spacetime discontinuities, we do this by solving the following sequence of problems: (1) the spacetime interface, (2) the double spacetime interface, or spacetime slab, (3) the unbounded crystal, and (4) the truncated crystal. For these problems, we present the following results: (1) an extension of the Stokes principle to spacetime interfaces, (2) an interference-based analysis of the interference phenomenology, (3) a quick linear approximation of the dispersion diagrams, a description of simultaneous wavenumber and frequency bandgaps, and (4) the explanation of the effects of different types of spacetime crystal truncations and the corresponding scattering coefficients. This work may constitute the foundation for a virtually unlimited number of novel canonical spacetime media and metamaterial problems.