There's an elevator hidden behind a false wall in the Zhent hideout that takes you down to the underdark stash. It's a much better place to fight the Minotaurs from plus you can see how their patrol makes more sense as guards from there.
As for why the Zhents are there, aren't they the major suppliers of surface slaves and goods to the underdark now that the trade with Thay has collapsed? (because would you buy food from zombie farmers, it's a bit icky, and probably contaigous)
