Last post, I talked about climbing Mount Fuji. Today, applying that experience to OSR D&D.
Time and distance: In nine hours, we covered a total of six miles of trail (plus ~8600 feet of total elevation delta). We were barely encumbered, pessimistically two stone. ACKS' rules would put us at 120' exploration movement speed, or 24 miles per day, times 1/2 for mountains. So the rules would overestimate our speed by a factor of two, about. It's possible that we're just that out of shape (though I suspect we're in as good a shape as your average L0 henchman; by 3rd Edition's strength standards, my mediocre deadlifts put me at about Str 9). Certainly naive application of Naismith's rule would argue that we should've taken closer to six hours (but there are a bunch of proposed corrections to Naismith's rule for a reason; notably, we were on roughly 15 degree slopes, which some of the corrections penalize further). It's also possible that the rules are for overland travel, rather than summiting a mountain, and that if we were just trying to cross a mountain hex to get to the other side, we'd've done a lot less climbing, and consequently our speed would've been a lot higher. This introduces some weird questions about moving around within hexes versus entering and leaving hexes. If I'm already thinking about hexes as being like 10' squares, and groups of hexes as being like rooms, that probably isn't something I want to worry about (and instead I might just increase the penalty on mountain movement). But if you're like the typical hard-simulationist ACKS hexcrawl DM, maybe that is something you do want to think about, especially if interesting things tend to be located at the summits of mountains (and of course they do).
One thought I had, reflecting on our covering four miles of distance in a full day, was having irregularly-sized hexes instead of changing multipliers, making the hex a unit of time rather than a unit of distance. So a mountain hex is actually smaller than a plains hex in space, but both take a day to cross. The problem is tessellation; maybe irregular, Voronoi-style maps would support this better (I guess there might also be problems in economics, if you care about population density per hex for taxation purposes). Or maybe, like Arneson, who also did "one day, one hex", you just don't worry about it.
Encounter Distance: I think the 2d6x10 yards encounter distance for Badlands would've been more appropriate than the 4d6x10 yards typical for mountains, given the mists and the steepness and general difficulty of seeing even building-sized objects at 200-300 meters. Having weather be a bigger determiner of encounter distance than terrain would be interesting, especially if you have typical / unusual weather for certain hexes or "rooms".
Foraging: There wasn't anything even plausibly worth eating above 2500m of altitude, except for one bird. Foraging penalty in high mountain hexes seems appropriate. The yamabushi allegedly developed systems of diet and cuisine for high mountains, as did the Taoists in China, but... I'm betting they foraged a little further down.
Hypoxia: -1 to surprise (loss of color vision, difficulty paying attention), initiative, attack and damage throws, proficiency rolls... morale? spell failure? Some people get incoherent and start repeating themselves, that could be bad if you say Hastur one extra time... And sure, you can get acclimated, but it takes a week or two. That would actually be kind of a neat mechanic, if I'm treating the day as the unit of wilderness time, that you can get acclimated within the timespan of a single adventure (and then get de-acclimated when you go back down to civilization for a month). And then if you have a mechanical reason to linger partway up the mountain for a week, you're really going to want to find or build a refuge there...
Combat: Seems like it would be really difficult for anyone coming up to fight someone higher up, especially with premodern weapons. At the very least there's an ample supply of rocks to drop or roll down on people, and the height advantage in melee on a 1:4 slope also means that if you're five feet away, you're at a foot and a quarter of elevation difference, which might mean that you can hit them in the head really easily but also means that you're going to have to defend your legs differently or have them cut from under you. Melee generally would be made difficult by the bad footing (gravel, irregularly-shaped rocks). Charging uphill seems impossible, and charging downhill seems foolhardy. Single-file-width trails limit the number of fighters who could be concurrently engaged without risking slipping and rolling down the mountain. Firing arrows up, you end up converting some of that kinetic energy into potential energy, much like firing up at the defenders on walls during sieges; the -4 "volley overhead" penalty seems like too much, but -2 might be right (this is also interesting more generally for firing ranged weapons at flying opponents with an altitude advantage). I could also see fireball and other big magic setting off rockslides.
Mounts: I'm not sure how viable riding a horse up would be. I didn't see any evidence of horses being used on Fujinomiya (though apparently they are used on the shallower-and-less-rocky Yoshida trail), and for the Himalayas local human porters seem to be preferred. If horses are supposed to be one of those pieces of kit that really changes the wilderness exploration game by increasing the speed of small, elite adventurer groups relative to mobs of infantry like most beastmen, I'm not sure what the implications of this are.
Wilderness Damage: This is something Tao of D&D was kicking around a few years back, and I think that it's a solid model of the realities. If hit points are an abstraction for your luck and vigor and ability to turn dangerous situations into no serious bodily harm, that's exactly what gets ground down by exposure and hunger and being altitude drunk, and then when you hit zero from these 1-2 points of chip damage you roll a (probably) not-very-serious wound from the Death and Dismemberment table, like falling over and knocking your teeth out on a rock in the trail or losing a couple fingers to frostbite. Doing something like this opens up some interesting design space. Weather obviously plays a part in determining how fatigued you get. You could even make the wilderness damage roll determine the weather - if you're in mountains in winter, the daily wilderness damage roll is 1d4 or something. 4 means it's a blizzard today, 1 means it's just cold. In the summer, it's 1d4-2 instead, maybe, so most days just being in the mountains is OK but sometimes you get rained on and you get cold. Forced march and night march (I recall from von Schnell's book an aside about making a night march and it being very lucky that they didn't lose anyone) give you a choice not just about rest, but about damage. Abilities like Endurance and Survival might reduce the amount of wilderness damage that you take by 1 per die, as might equipment, and being in a sanctuary reduces it a lot (say 3 points, in our mountain blizzard example - it's still going to be cold, you're still going to want a fur cloak, but you can get by if you stay inside). There are a couple of annoyances here, of course - building coherent systems is hard, hit points in OSR games are very low-resolution, I don't want to track HP for a bunch of mercenaries, and handling a die roll for damage every day (er, wilderness turn) sounds like a hassle (especially when you start considering crossing multiple hexes per day and changing terrain types and whether marching inflicts damage, which it seems like it should, you're less likely to twist an ankle if you're sitting around all day).
Equipment Loss: My father almost broke his glasses. I destroyed the heel of my boot. A couple of times we got our poles stuck in rocks and could've broken them if we were clumsier. Obviously these are recoverable once you get back to civilization, but out there these could be quite bad. I feel like destruction of mundane equipment is a totally reasonable effect for a wilderness trap or mishap, and one which (particularly if aimed at rations or water) might really be felt. Some items of equipment, like optics, might be fragile by nature and "first to fall" to such mishaps. This might also be true of horses, which are notoriously fragile among work-beasts and easy to lame.
The Weight of Wealth: A 100-yen coin is five grams, about an inch across with a little hole in the center, and made of something probably less dense than gold (copper, the primary component, is about half as dense as gold, it turns out). But 1000 5g coins is 5kg, which is a bit shy of the British definition of stone but pretty close. So if you're running ACKS and coins are 1000 to the stone, gold pieces are probably about half an inch in diameter, maybe 3/4 of an inch with a hole in the middle.
Pilgrimage: An obvious use for mountains, and one used by both Morrowind (pilgrimage up Mount Kand) and Skyrim (to High Hrothgar). Turns out Fuji used to have quite a widespread cult in Japan, with over 1300 shrines built out of rocks from the mountain. Next time you want to make religion in your campaign less boring, consider the Good News of Volcano-Gods.
Volcanic Craters: They're big! And totally hidden from view until you reach the summit (unless, like St. Helens, the whole side of the crater blew out during an eruption). I think Fuji's was about a quarter mile across. You could hide a lot of stuff in there - goatman village, volcano cult temple, tarrasque...
Torii Gates: They mark the boundary of sacred ground. You're supposed to take off your hat and bow when you pass through them. It would be interesting to use a similar mechanism to provide information to players, of symbolic gates to mark the boundaries of civilization and peace (separating the temple at the summit of the volcano from the mountain wilderness, say) or to invert them and have them mark the boundaries of the otherworld, the supernatural wilderness, where spirits and demons are active.