"It seems that the humans are losing hope. Today they are sloppier than usual, spreading their forces too much to be effective." Sukhet the Banshee said on his communication amulet.
"Agreed. This way they'll accomplish nothing but being swept away as soon as our main force wakes up." Angorn the Lamia replied.
As time went by, the conceit disappeared from their voices, replaced by genuine worry. The humans were just pretending to attack the twin fortresses, using that time to set up something.
The undead generals' problem was that they had no way to stop them. The humans were employing just enough manpower to force them on the defensive, but nothing more.
If the undead deactivated the dimensional arrays sealing the cities to move their troops, they would also open themselves to an invasion. The same would happen if they sent part of their main force outside.