Strategic use of Demon Portal
 

I've noticed some battles start with the enemy in just the right layout so a well placed Demon Portal will totally block in a powerful melee unit keeping him out of the main fight for a bit.

Also with the mage class higher magic, when the terrain is just right 2 Demon Portals right at the start can create a brutal choke point working against the enemy. I've taken out huge armies with no effort using that benefit.

Works nice to block off those distance attacks over 1 square as well (green dragon, whip attacks from demon, pirates). If used with a mage it also help to make trapping easier. Block off 1 route with portal, place trap in path they now HAVE to take.