  Band engineering in twisted bilayers of the five generic two-dimensional
Bravais networks is demonstrated. We first derive symmetry-based constraints on
the interlayer coupling, which helps us to predict and understand the shape of
the potential barrier for the electrons under the influence of the moir\'{e}
structure without reference to microscopic details. It is also pointed out that
the generic constraints becomes best relevant when the typical length scale of
the microscopic interlayer coupling is moderate. The concepts are numerically
demonstrated in simple tight-binding models to show the band flattening due to
the confinement into the potential profile fixed by the generic constraints. On
the basis of the generic theory, we propose the possibility of anisotropic band
flattening, in which quasi one-dimensional band dispersion is generated from
relatively isotropic original band dispersion. In the strongly correlated
regime, anisotropic band flattening leads to a spin-orbital model where
intertwined magnetic and orbital ordering can give rise to rich physics.
