How deep is the Nave?

[Froge]

Castle Brennenburg is gargantuan, but I'm pretty sure there are some castles in real life that are of similar size...up until the Nave and its connecting areas. The Choir is so massive that it looks more like a pavilion, and all the weird roots sprouting out from the ground makes me believe it's the lair of some eldritch abomination. It would also take forever to dig out all the rock to build the massive room that is the Chancel. I like Alexander's line "[The Inner Sanctum] lies beneath the very stone of Brennenburg," to really illustrate to the player the sense of going down a "Dark Descent."

How deep do you think the Nave is?