Bug or my misunderstanding?

[Beorn]

In my Caster of Magic for Windows game, I lost a Great Lizard to an AI, which turned it undead. When I attacked with invisible nighblades, the undead great lizard appeared unable to attack my units.

Checking his "card" it appeared it had immunity to illusions, so I would have thought it could attack. But it started the battle headed straight for my invisible units (but so did his sprites, after all, my units have to start in that location) but after a couple turns, the Great Lizard just took evasive action off into a corner. And even when cornered, never attacked, although doing some damage in retaliation.

[Seravy]

Enemy units that move faster than yours will try to avoid your army if they are far outnumbered and know they will lose. It was probably that.

[Beorn]

I guess that must be it. I had thought he could beat me, but maybe I am wrong about that.

Thank you, in any case, this is an incredibly good game.