When the
Defend window closes (see '
Attack' in the
codex for details), If only a single
enemy unit uses the Defend
ability, the
Defender is removed from the
fight and the original attack
target is reinstated. Defenders still arrive one at a time, but if
there's only one, the defense effectively is negated (though that one defender still got to
tap and
move to the attack's
location).
