Hi SandyBean,
first of all - thanks for the nice words about
Underground Fortress !
I am always glad to get feedback from players...!
Your question has been pretty much answered by Semi-Native and Rose (thanks, folks)..... who are definitely both UF specialists.
But one thing you said strikes me as odd:
You mentioned you had already tried to access room 15099 from all sides, without any luck ?!

:o
This can't be true......because as you see, room 15099 is not closed off at all at its right side....which should lead you to the conclusion that at least from there it should be easy to access !!?!
Also never forget that I am a big fan of
secret passages.....a hint that will help you also in other corners of the Underground Fortress.
Alas, I'm sorry to report, in spite of all your efforts there won't be too much pecuniar recoupation to be had in 15099...... but it's still worth visiting !