Thanks for the fast reply, although that's a bummer.
So basically any resource on a mountain is locked away until the medieval era then. Considering that stuff like stone, obsidian or copper is needed pretty early on, I'm wondering why "Usable Mountains" is often recommended, it seems it hampers your game more than it actually helps. Do people like the 3 hammers they get from the tile? I'd much rather have the resources early, when starting from Prehistoric at the time you enter Medieval the game is mostly decided on the slower game speeds anyway.
Are there any big benefits from having workable mountains that would balance out that (in my opinion huge) drawback?
Without Usable Mountains, peak plots are entirely useless, you don't get a thing from them with any tech, just like vanilla. At least...I think that's what the game option does.
A very good solution I've been seeing recently is from the Civ4 Colonization mod Religion and Revolution [
https://forums.civfanatics.com/forums/civ4col-religion-and-revolution.455/]. In that mod (or maybe it's also in regular Colonization, I don't know I've only player RaR these past months), mountains are impassible to all but a few units. Once they have a road however, built by a pioneer who CAN pass into the plot, any unit can pass them (great for getting armies past the Rocky Mountains or the Andes). And of course the bonus is linked, not that it does anything in Civ4Col but it would matter in C2C.
Ideally of course each different route type would enable different unitcombats to enter. For example, a trail would only let recons through. Mud path would let melee/archers, roads would permit mounted, paved for catapults and other sieges, tanks might need railroads, etc. Early on though, just having a route and a basic improvement might be enough to access a resource. Or maybe not. A copper deposit for example. Sure, there's copper in those mountains, and sure a few rugged miners from the city might sometimes trudge up there to dig a few holes, giving the city a few more hammers of yield, but without the transportation and logistical infrastructure required to build and maintain difficult mines in those area, including various forms of specialised labour, sturdy wheels and pack animals, and nourishing food, you could not honestly say that those few hammers are enough to allow every city in your empire to produce copper wares and outfit your armies of axemen.