"It says here 'And ye shall dig a maze in the earth, that adventurers may venture therein and wonder Who Builds This Stuff Anyway?' I think that's supposed to be us."
The ruins turn out to have been built pre-ruined, just because a nice set of ruins is good for the area's ambiance.
Zig Zagged: Under the dust, the ruins are full of Lost Technology, but it's all broken down, so it's only purpose is to lure adventuring parties to the monsters who set up shop there, but the Gadgeteer Genius is able to repair some of the machinery, but the computer systems that run it are based on old dungeon-crawling RPGs... (continue zigzags ad nauseam)
Averted: The ruins make perfect sense; they look like a ruined watchtowers, sewers, citadels, temples, or whatever.
Enforced: "OK, and we need another dungeon here. It's just for a Fetch Quest, so don't worry too much about it."
Lampshaded: "What was this building for, anyway?" "I dunno."
Invoked: "We'll need some tough adventurers to help us on the road ahead. Let's put up some fake ruins and hire whoever discovers them."
Exploited: The Precursors set up fake temples with fake gods for outsiders and pillagers. Thus they'd risk life and limb to grab their silly little pyrite idols and wooden totems while leaving their real, better hidden, and far grander ones alone.
Defied: "You've been playing too many video games. Ruins always served a purpose of some kind when they were built. If you examine the placement of the pillars here..." (some time later) "...which, when all put together, clearly suggests it's a Precursor temple to Crystal Dragon Jesus."
Discussed: "You've been playing too many video games. Ruins always served a purpose of some kind when they were built; we're just having a hard time figuring out what this building was used for."
Conversed: "It's fine as a video game level... but seriously, who'd actually build something like this?"
Deconstructed: As it turns out, the ruins make perfect sense: They were built to attract adventurers thousands of years after their building, so the monster living inside can have some nice dietary supplements on an occasional basis.
Reconstructed: Same as above, except the monster was a good guy all along and designed the dungeon to attract the Chosen One.
Plotted A Good Waste: The ruins make no sense as buildings because they were never buildings to begin with. They look like ruins to disguise that their origins are much more recent, and they're currently in use by someone with big, nasty plans.