"The Ancients were capable of wondrous things, but they often made mistakes, and 'dungeons' are the outcome of those mistakes" is a common conceit in dungeon-crawling fantasy, but the Ancients' fuckups are typically framed as products of hubris or madness. I want to see a setting where they messed up for the same reason that real-world engineering and public works projects often come to horrifying ends. The safety reports were suppressed because the architect was somebody's cousin. The plans clearly called for unobtanium rods, but a malfeasant contractor swapped them for mudanium and pocketed the difference. Somebody got sick of having to re-summon the hellgate each dawn and propped it open with a shoe.
The warding sigils would have detected the problem, but they were so overtuned that they were constantly throwing false alarms, so eventually one of the assistant thaumaturges simply disconnected them and forgot to tell anyone they'd done so.
