Also do enemy units have to attack each turn?
Yes, they DO attack (if an attack is possible) Have FUN.
From the rulebook page 15 under ENEMY ACTIONS:
Roll once on the Enemy Tactical
Movement chart to determine the
Movement Orders for all enemy
Units at the start of each Enemy
Use the Tactical Move counter to
mark the die roll as a reminder of
Move each Enemy Unit, then
resolve its attack, before going
on to the next Unit. You can
resolve their actions in any order.