The Caverns of Thracia by Paul Jaquays is a good example of why Judges Guild is remembered so fondly by so many of us who started gaming in the 70s. Published in 1979, Thracia is both a large dungeon and a campaign setting in its own right. While perhaps not large enough to be called a true "megadungeon," the four levels of the caverns are nevertheless expansive and filled with a wide variety of humanoid factions -- a few of them mutually antagonistic -- which contributes greatly to the feeling of dynamism the module evokes. This is a "living" environment that puts paid to the notion that old school dungeons are static places with monsters statically side by side without any interactions between them.
Even more significant, from my perspective, is the diversity of environments within this module. Firstly, there is the Lost City of Thracia itself, a surface ruin that is the start of the characters' explorations. Beneath it lie four levels, several of which have sub-levels and special room complexes that are only reachable through certain areas within the larger levels. In addition, there are multiple connections -- shafts, chutes, stairs, and other more exotic means -- that contribute to the maze-like feel of the entire place. The Caverns of Thracia is very non-linear; there is no "right" way to explore its depths and no central "set piece" locations. This is a style of dungeon design that we just don't see anymore and that's a pity, because I think gamers are missing out on the unique pleasures of exploration that can only come when the dungeon environment itself is as much of a challenge as the monsters, tricks, and traps contained within it.
I have a lot of fond memories of The Caverns of Thracia. It was one of the few Judges Guild modules I ever played in my youth. My friend Mike's older brother had a copy and more than a few of our characters died horrible deaths while exploring its labyrinthine levels. But we had fun doing it, because the pseudo-ancient Greek atmosphere of the place, combined with its memorable encounters -- like the cult of Thanatos, the incarnation of Death -- spoke to our dreams of being Indiana Jones. I was eventually inspired enough to create my own Egyptian-flavored version of The Caverns of Thracia and, while that dungeon thankfully hasn't survived, it was an important moment in my education as a budding referee. I have Paul Jaquays and Judges Guild to thank for that; I wish similar lessons might be imparted to today's generation of gamers.