Maldroth, you win an official No-Prize from Goodman Games for spotting the second piece of unofficial official errata from Castle Whiterock.
After conferring with my co-author, several maps, my own copy of CW, a few Word documents, and the Oracle of Delphi, we've determined that there kinda is no entrance to sub-level 6B. But only kinda.
Check out your Book of Maps...on the front cover is a cut-away map of Castle Whiterock and the mountain. Sub-level 6B is shown as connected to level 6. Now look at the map for level 6...there's a tunnel in area 6-7 that leads nowhere...it just ends. The entrance to sub-level 6B is supposed to be in that tunnel, protected to some degree by the stonebore ant queen and her soldiers.
Here's what happened: when we were putting together the jigsaw puzzle that was the levels and sublevels of CW, we knew where sub-level 6B was supposed to go, but it never got physically linked on the smaller maps...a rare lapse between my co-author (who wrote 6B) and myself (who wrote 6). That probably had to do with level 6 being the first thing I wrote and 6B being one of the last things he wrote.
The next post gives the specific information needed to insert sub-level 6B into the dungeon (or, as with everything else in CW, you can change it to better suit your group and game).
Co-Author: The Almanac of the Endless Traders, DCC #13, DCC #29, DCC #49, DCC #51, DCC #52, DCC #63
Author: DCC #55: Isle of the Sea Drake, DCC #61: Citadel of the Corruptor, more to come....