Undead fortification?

[jejeba86]

Now that we have some ways to lower an enemy fortification, I have a doubt about the immune mobs, e.g. undead.
Are they just immune to sneaks and criticals or do they have some amount of fortification that we could eventually drop to make them vulnerable?

[Neverwinterer]

Undead used to have 200% fort. If you can lower more than a 100% then you can start making crits.