The in-comic explanation is that Dorukan--the mighty wizard orchestrating all the defenses of his epic dungeon/gate defense--vanished a few months ago. It's the same reason Girard's pyramid was mostly defenseless.

If that isn't enough of an explanation, there's more from Start of Darkness.
Spoiler: Involves Start of Darkness
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Xykon killed Dorukan outside the tower and subsequently routed his forces, save those related to the Amulet of Dorukan for which Xykon had no interest (Remember, touching the sigils was Nale's plan, to control the 2nd ed monsters). Since that was only a few months before the comic started as indicated by Redcloak training the new door guard, (which was evaded by Roy's OotS in the bonus strips at the start of "Dungeon Crawlin' Fools" {print} ) there may not have been enough time to bother setting up appropriate defenses. Or perhaps Xykon was so smug he didn't think he'd need the defenses, or to even hold the dungeon for long... the seal which inevitably exploded him was the only thing stalling his completion of the Gate ritual.