  Maxwell's equations with massive photons and magnetic monopoles are
formulated using spacetime algebra. It is demonstrated that a single
non-homogeneous multi-vectorial equation describes the theory. Two limiting
cases are considered and their symmetries highlighted: massless photons with
magnetic monopoles and finite photon mass in the absence of monopoles. Finally,
it is shown that the EM-duality invariance is a symmetry of the Hamiltonian
density (for Minkowskian spacetime) and Lagrangian density (for Euclidean
4-space) that reflects the signature of the respective metric manifold.
