  We revisit the calculation of vacuum energy density in compact spacetimes.
For the general case of $k$ compact spatial dimensions in $p+k$ dimensional
Minkowski spacetime, we calculate the Casimir force on a piston placed in the
presence of external electromagnetic fields. We observe that while the presence
of a strong external magnetic field reduces the intensity of the Casimir force,
the presence of an electric field enhances it instead. We discuss various
physical cases, and also derive the particle production rate in the presence of
such external fields in compact spacetimes. We observe that the pair production
rate is enhanced in the presence of a piston.
