[WhYdOyOuThInKtHaThApPeNeD]

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.