It's a much-requested feature...
Clearly, there's a certain amount of interest in such a feature. A number of DF-lite clones and similar games like RimWorld and Prison Architect do not do z-levels. So, this niche that certain players long for largely goes unfulfilled.
...there's plenty of reasons both for and against them, but the important ones driving the decision for me is that fortress layout and efficiency is a lot more important when you're constrained to a single level and it's very hard to convey information from multiple levels with a top-down 2D view.
Actually, if you think about it, fortress efficiency is only truly maximized if z-levels are involved. If you can have your military barracks, workshops, etc. near your sleeping quarters, booze stockpile and dining area (i.e., vertically), then the amount of distance that your dwarves have to walk to eat, drink and sleep is greatly reduced, resulting in significantly less wasted time and more work done.
Similarly, consider having your resource stockpiles immediately under or on top of your workshop and forge levels, or having your food stockpiles and/or butcher and cook on the level above or below the dining area.
To sprawl horizontally means vastly longer walking distances. Often, this also means having a much larger wall to defend from hostiles, too.
That said there may be some limited version of it via portals and entrances to other maps from your main map.
Well, at least that's something.
Yeah, I think z-levels work in DF because you can count on the dwarves to just go about their business without much direct input.
But, don't our units in King under the Mountain also go about their business without much direct input... or not?
Do they require a lot of micromanagement? If so, then that
would be a good reason not to add z-levels.
More than that, DF makes absolutely no claim to being user friendly in terms of display/whatever. That's why we have third party utils like Dwarf Therapist.
More than
that, DF has mods and other add-ons to make keeping track of multiple z-levels easier. TwbT-based graphics sets like DungeonSet and Meph Tileset will display multiple z-levels at once by default, with lower z-levels appearing
shaded or darker to make it clear that they are lower. And visualizers like Stonesense make it easy.
I'm not sure what would be required to add visualization of z-levels to King under the Mountain. However, it is already a visually-oriented game with what sounds like a user-friendly interface, so I doubt it would be too difficult.
Whilst you miss out on the scale of the mountain without z-levels, I think it's easily outweighed by the ease of use factor.
I disagree. But then, I guess it's a matter of preference.
The lack of z-levels limits certain aspects of gameplay, such as how we may design our fortress and how far away from our fortress we have to expand in order to find more minerals to mine or to gather resources. (DF allows players to even grow crops and mushroom trees underground.) Z-levels greatly expands possibilities in terms of strategy and base design. Though, yes, it does add complexity. But, without them, players are forced to
sprawl out horizontally in a major way in order to expand, which has it's own problems.
Not being able to dig down for minerals might be my biggest gripe, though. RimWorld, at least, has the
Deep drill, which allows players the benefits of digging down for resources without actually having z-levels. It's just a structure that must be built, supplied with power, and operated by one of our units in order to gain resources over time. The type of resource and the amount available is random.
Myself, I'm particularly fond of CuproPanda's
Quarry mod for RimWorld, which allows the player to designate a large area for strip mining, resulting in random resources. It requires a lot of work or a lot of units, though, which keeps them busy.
Even very old strategy games had something similar. For example, remember Warcraft 2? You could have your humans or orcs claim a mine, which was a source of gold or whatever. (I also seem to recall spots for petroleum to be gathered for ship building.)
I'm also reminded of Stronghold, an old (1993) D&D strategy game. Again, the player can claim certain spots for mines, hinted at by odd-looking rocks on the surface.