General knowledge regarding the Caverns of Borog.
Borog was a Deep Dragon who lived within ( indeed carved a network of) tunnels and caverns that came to bear his name ever after. Long after the Dragon had been slain, his legacy of warrens lives on, miles and miles of caverns, tunnels and spacious vaults filled with grottos and fungal forests.
Many are the creatures that live amongst the Dragon's ancient home, most notably Troglodytes and Derro, the former in natural formatins, the latter having carved out small fortresses for themselves. It was deep within this network that the party of Yrtchull was ambushed, there on some business of Enraela's (Yrtchull's hated older sister). As well, teretha and Rith'Arnis had been following a lead regarding suspected Illithid activity in the area, possibly in link to the Derro, for that depraved race has been known to ally themselves with the flayers of minds.
The Castle that you are currently within is owned by a mysterious Mage, though none of you have as of yet seen him/her. Troglodytes seem his regular troops, though you have encountered extra-planar creatures as well, so he obviously has a command of the Summoning arts.
Both Teretha and Nurin know their way through the castle back to the front Gates, and hence out into a fungus forest that sits outside the place, one of the tracks that serves as a road through the place not too far off.
The Drow city of Thelaru'acli lies some 50-70 miles South, along many different intersecting roads, some small and less travelled, some more open and the causeway through which much traffic goes.
OoC:I will do a bit of editing to reflect what Teretha and Rith'Arnis know of the castle from my full maps. I'll try to get them up tomorrow, after I get some sleep.