Great topic!
Douglas Cole's
Hall of Judgment features a number of magical locations. Without too many spoilers, I particularly liked the idea behind the Nordalf warrens where there are passages that cross into the fae realm:
Quote:
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Originally Posted by Hall of Judgment, p. 55
It is faerie home turf, the quarters are very tight, and the noršalfar can use passageways denied to mortal folk without the right lineage, gifts, or spells.
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In effect, this means that the fae denizens of the warren have access to numerous invisible secret passages that the PCs cannot enter.
I launched my Nordlond campaign with a small version of one of these warrens and have used the idea multiple times since then. The party ended up in a bargain with a satyr who led them into its tree (a modest exterior leading into a massive wooden cathedral within). It had multiple doors that led to different trees throughout the Endalaus Forest (and some opening into deeper fae realms).
I'll try to share more examples as time permits.