2 Born here
3-6 Made here (1-2 recently 3-6 in ages long past, now best forgotten)
7-8 Wandered in accidentally through a spontaneous entrance from one of the usual places. (1-3 old section of a mundane library where no one's been recently 4 wizard graveyard 5 magic forest 6 anywhere constructed with or with furniture made of wood pilfered from a magic forest (or stone pilfered from a wizard graveyard)*) 10% chance the passageway is still accessible somewhere on this level.
9 It's always been here
10 The PCs are somehow responsible
*not a recommended building material