This is something I noticed recently. This unit says "When this unit attacks, target unit must defend if able." But it does not say the target unit must be chosen from the defending zone. So can I make any of my opponent's units defend when attacking with this unit?

In most cases, no. A unit that isn't in the zone that's being attacked isn't able to defend an attack on that zone, unless it specifically has that ability.

Thanks for the reply.