Why was the Dungeon of Dorukan so easy?

[Metastachydium]

The runes were purely magical - Reddie and Xykon are equipped to understand pure magic. Neither of them have many, if any, demonstrated skillpoints in Medicine or Knowledge (Nature), however, so it is pretty unlikely they would figure out why Xykon lost his powers, especially since Redcloak didn't, without being explicitly told why.

Mhm. I suppose that's why throwing live goblins on it was the best strategy of trying to understand their nature they could come up with in all that time.

Let's take a Lawful view - would Redcloak accept the terms presented to him immediately by a Devil, without suspecting something? Heck, Redcloak didn't even accepting the terms of a Lawful dwarf without suspecting something a few strips ago.

This is not a strictly Lawful issue.

This is a Lawful issue. Redcloak seems to have taken Improved Paranoia somewhere down the line, so I don't think he's the best example you can come up with here. HIs (presumably mostly) Lawful Evil/Neutral hobgoblin minions would readily accept the terms presented (if accepting as a given that their (at that point) openly racist Supreme Leader's orders must needs be the best course of action because they come from an authority figure is anything to go by).

Not at all. I'm saying that powerful, free-willed undead, like Great Wyrm dragons, are rare in this setting. I believe I have been very clear on this point.

Like I said, they are not quite so uncommon that one would not expect them to show up somewhere important (mind you, this universe runs on narrativ rules, and its inhabitants are aware of that). Also, necromancy is a well-known, widespread form of magic in the setting. Even Julia was taught necromancy in her school. A necromancer with well applied undead minions (including potentially powerful ones: Redcloak was 16th level (far from epic) when he created three free-willed undead lieutenants for himself during the course of a single day) controlled remotely can be quite a threat in and on itself and that one of these would want to assault a Gate is not quite as absurdly improbable as you'd seem to think.

You mean the Gate that Redcloak and Xykon wouldn't even have known was there if some other goblin hadn't told his god?

I mean, that was your argument against Tarquin's army, right? He didn't find it himself so it doesn't count?

It was mostly „hidden” by the fact that very few know about the Gates in the first place. Lirian did not go the extra mile to really hide it. I mean, Soon's Gate was basically disguised as a pretty ornament on a throne. Girard's Gate was hidden in a lead-lined pillar that was hidden in a pyramid choke full of illusions that was hidden by further illusions in a hidden valley hidden in the middle of a canyon somewhere in a desert.
Lirian tied her Gate which looked liked a Gate to some trees.