I once genned a world in which all the human towns were gone, but there was a necromancer's tower that my adventurer consistently started at. (This was vanilla, by the way; otherwise it wouldn't be wildly relevant.)
I tried several times to break into that tower, but never pulled it off and I think my adventurers' corpses were added to its defenses. I realized after scrapping that world that I hadn't tried not only sneaking but also lying down to crawl under the zombies and past them into the tower (instead I'd been trying to beat up the handful between me and the stairs without getting caught and clobbered); oh well, I could probably figure out how to get a world with no human towns and only one necromancer tower again if I wanted to try doing that right.
The whole game preference for starting in a human town if possible does plop one down in an oddly constricted area of any given world, I've found. I wonder though whether there's been any !!science!! on what the game does when one is out of towns (is necromancer tower the only option, the preferred option, or an equal with bandit camps and tombs and whatnot?) or, for that matter, out of absolutely any civilization site it would normally start at (I believe one could still play as a human outsider? have I got that right in the first place?).