Semantics indeed - if that's what you mean by thieves guild then we're talking the same thing. Just wanted to be clear that there are crime bases not centred around thievery. In real medieval times it's hard to imagine any place called a thieves guild to be honest - such dark dens would I guess normally be rooms at the back of or underneath legitimate businesses likes pubs and restaurants.
Y'know, it'd be cool as a particularly powerful and evil character to have your own dungeon, complete with random spawning monsters etc. Maybe even a moat of chaos pirahnas around your main base on the bottom level