Likely though, if the father was human then the half-elf was raised in the elven city.
Or s/he and it's mom were banished and lived a hard life on the run, possibly looking for some vengeance (plot hook?).
Also, they are secretly nobility, like every other adventurer.
Since I want an actually city for story purposes, I am not going to do the amalgamation of hamlets magically connected to each other. But seriously, brilliant idea, and has a strong RP and mechanical value to it.
The option that I am leaning toward was independently discovered by Rot42. Use basic screening and illusions to foil easy and accidental detection (Forbiddence, thin sheets of lead, running water, Silent Image), and then use Contact Other Plane, Divination and Commune to discover if the City's existence will be discovered by any non-elf, and then Scry'n'die/Memory Wipe. unlike most other methods, which are primarily defensive (hiding the city), this is aggressive and likely spur on a adventure by its own right. (the adventurers return to the realm of their lord, and as they enter the court, three masked humanoids pop in, kill the lord's bard, and then pop out.)
