Why was the Dungeon of Dorukan so easy?

[Peelee]

It's not quite the same thing. Unlike Lirian's virus, the sigil is not an active effect. It's a bit more difficult to tell what it does and how it does it.

It's not; I doubt they would have figured out how Lirian accomplished it at all if she hadn't told them.

Yup. Like I mentioned (in response to dancrilis), I also think that could become an issue. Dancrilis is, however, probably right that at this point we cannot be absolutely certain there is no catch in that.

I agree, but until that catch is known I have no problem standing by it. I fully expect to change my view if the catch changes things.

It's not a brute force timesink in the sense Kraagor's Gate seems to be one. It's a puzzle. It takes a little more than just time to solve it: two Evil people have to kind of understand the mindset of a Good wizard to get past it.

The same two Evil people who understood the mindset of Good paladins?

Uncommon or not, people in Stickverse know that liches, vampires and the like exist, and know one when they see one, invariably. Not warding the Gate against attacks by powerful undead creatures is not by any means the same thing as not foreseeing that some caster can use an obscure homebrewn spell only one (now dead) person ever knew to cast on someone they didn't even know to have been their relative, and not making arrangements to prevent that. I mean, they were a secret society. Few knew they even exist, fewer yet knew where they are, and none other than them knew their numbers and the fact that they were all related. Lirian's sloppiness is a stupid mistake

No more stupid than Soon not warding his tower against a Great Wyrm dragon. Which is to say, not stupid at all.

It's easy to march on something once you know where it is (which Tarquin didn't until Nale and the Order have found it for him).

Why, it's almost as if all the Gates had "security through obscurity" as an additional measure.