Castle Whiterock was pretty awesome, especially for 3.x era. I agree completely that Whiterock took the idea of quests, subquests, milestones, etc to a very high art; well worth reading for inspiration. My group was obsessed with ultimately finding the lost shrine of Justicia. (But they died...)
Comparing it to a classic megadungeon (let's say Temple of Elemental Evil, something most people are familiar with) - you can see the levels are not as large horizontally, they're fairly linear, there's typically only one access point per level, one exit down per level, few/no major highways in-out of the dungeon, etc. But the encounters are very well-designed and there's very good details on making the dungeon 'respond' to incursions via 'high alert status' and restocking.
I can say its the only 3.x era adventure I ran (right up until my group's
TPK).
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