Huh, I guess it does kind of look like a Rimworld map. One of those built-into-a-mineshaft kinds of bases, maybe.

Thank you! I love making it so that each room tells a story of some kind, and there's a definite reason for it to be there. I guess now that it's been brought up, I can call out a few intended spaces. Some of these details I extrapolated from the adventure Murder in Baldur's Gate:

• The Festhall: A seedy tavern where shady business deals and racketeering schemes are made, whilst the bartender and their staff look the other way. To satisfy the demands of high-paying patriars and other nobles of interest wishing to do business with The Guild here, they stock a large selection of rare wines and ales, especially those known to be from exotic locales like the Underdark and The Moonshea Isles.
• A room for Alfrus Manyblades' black-market selection of deadly weapons for sale. He notoriously will sell even the deadliest of implements to anyone; no questions asked. The Guild has been very pleased with his services, and have offered him a permanent setup in the Undercellar to help facilitate their mutually beneficial relationship.
• A room for Vug Gorkul, the refined half-orc herbalist brewing potent potions and poisons in his small, yet well-equipped office. Visitors would do well to not touch the curious plants he grows there, beautiful and unusual as they may be.
• A desk for Nasparl Nintanter, an eyepatch-wearing half-elf selling makeup, wigs, and other elements of elaborate disguises. For those particular with their needs, he invites them back into his "showroom." There they will find outfits in a wide variety of fabrics and styles, which allow their wearers to pass as a Flaming Fist watchman, a well-traveled sea captain, or a foreign noble attending a regal gala, among endless other possibilities. All of course come with pre-stitched hidden pockets in their linings, for whatever you may wish to sneak past even the most meticulous body search.
• A secret passageway used by Ribbons and Nine-Fingers Keene to transport high-value goods and political prisoners between the street level and the Undercellar's inner chambers. The passageway also serves as an emergency bug-out bunker, in the unlikely event that the Undercellar becomes overrun with Flaming Fist soldiers, murderous cultists of Bhaal, or a self-righteous adventuring party harboring a vendetta against them.

Edit: a few minor extra details