Ties in with previous suggestions about chimneys in that their smoke would be visible for some distance.
Unless the chimneys were vented into an existing volcano . . . or possibly behind a waterfall, hiding the smoke in all the mist.
Anyone know some genuine examples of hidden settlements/fortresses? I've only been able to turn up closed cities from the Manhattan project and such.
None of these are really Dwarf Fortress levels of engineering, but there's Lalibela, cave cities in Turkey, Navajo cave cities, the Mound Builders, arguably Petra and viking earth lodges.
http://news.yahoo.com/fabled-lost-city-discovered-honduras-ecological-wonder-archeologist-155548884.html
http://weburbanist.com/2007/10/15/7-more-underground-wonders-of-the-world-lost-caverns-and-cities/
Archaeological ruins are nice, but a more apt comparison would be places where a decent number of people actually
live, day in & day out. The American NORAD complex, for example: It's dug under a mountain with few points of access, it's heavily guarded & (one must assume) most of those guards are unseen, and most importantly, hardly anyone knows where it is (with any real precision), or even what it really looks like.
Now, from a game-play perspective how would this be calculated?
I'm figuring 3 main ways. The first is the most obvious: Large-scale dwarven construction that's visible from miles away. For every Road or Construction on an Outside, Above Ground tile, it calculates a running total of [what % of the tiles on that z-level are empty space] * [a modifier for the tile's altitude "above ground"] * [another modifier for the material of the construction: greater than 1 if it's reflective like glass or metal, smaller than 1 if it's of unfinished native materials, like rough boulders]. Add up those totals for
every tile of Outside, Above Ground construction, and you have the fort's overall Conspicuous Index. In forts not
trying to be secretive, this would very likely be so high as to render the other two calculation methods irrelevant.
The second method is to consider aboveground
movements, and their byproducts like footprints, as seen by intelligent creatures that either have some nearby elevated vantage, point, or can outright fly. This would be calculated about once a month, The entire aboveground embark site would be considered in this calculation, which would simply be a calculation of what % of the tiles show evidence of sentient activity. Actual dwarves & their livestock would have the highest rating, followed by outdoor farm plots & garbage dumps, followed by recent (in the past year) logging, followed by paths worn in the dirt from frequent use. Each embark tile (of which a default site has 16) would be broken into quadrants, and if any of those 64 quadrants exceeds a certain benchmark for visibility, the traffic at your site is considered "noticeable," and your neighbors can see you: not only do they know that there's someone home, they know roughly where to look.
The third way is for each civilization to send out actual scouts, or at least hunters, on a semi-regular basis. These visitors would have a "notice radius" dependent on their Observer skill, and how egregiously out of place certain things are. A marble lighthouse? That's guaranteed to be detected. A pig hoofprint where there should only be goats? Not so much. If the visitor sees something that shouldn't be, and lives to escape the map and tell the tale, more dedicated searchers, like a scouting party or an ambush, might be sent to find your fort's entrance. Expect sieges to follow.
You also have to consider word of mouth.
Not to the extent you depict. If a merchant caravan from the Mountainhome is met by a guard in a head-to-toe ghille suit who silently leads them to a trapdoor with a bush growing out of it, it's a very safe bet that the merchants
understand that this place is a secret, and that they
shouldn't go blabbing its location to their wife's friends' tennis partners. Dwarves of your own civilization aren't going to turn you in (at least as long as you remain loyal, of course). Even foreign merchants have an incentive to hold their tongues: As the only members of their civilization with access to your fort, they have a civ-wide monopoly on the customers there. If they blab, they lose that monopoly, and if your fortress should happen to fall, they completely lose ALL ability to trade there.
Of course, this entire idea is utterly
irrelevant for forts that have already been discovered by all of their neighbors, and thus are probably also known to any wandering megabeasts as well. It would be effective only in the early game, when your computer has plenty of FPS to spare for making those visibility calculations. Some people might not even WANT to rely on stealth in the beginning. But on the whole, I think it would be an improvement to bring back the early goblin attacks . . . to locations where the goblins have already actually FOUND something worth attacking.
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