Ravenloft could be called "dark fantasy". The overall feel is a bit more Renaissance than Medieval, but it can be pushed anywhere from Medieval to Enlightenment era.
Done with care, Ravenloft is very much about the heroes. It's up to the DM whether or not they can actually end the curse of any of the lords. Even if the curse can't be lifted, though, the PCs are the light that prevents the land and the folk from being completely damned. Sure, the Lord of a Realm may "live" on to do evil another day, but that's not any more defeatist than a setting that always has another villain waiting in the wings to replace the one that the PCs just offed.
Also, because the BBEG is usually following a script and the land hates the Lords as much as anything else, the PCs have the chance to defeat some pretty potent opponents at a pretty low level -- it's perfect for the idea of bounded accuracy in 5E. The key for the PCs is just to be smart enough to not stand anywhere they're going to take massive damage, and run when it makes sense.
If I were to run a Ravenloft game, I might give them a bit of freedom, early on, in moving between realms, but they'd eventually find one lord that really resonated with them (maybe you know your group well enough to start them there). That lord would be the focus of most of the campaign. If the game lasted to 20th level, I'd probably give the PCs a real chance of freeing that land from the curse and being expelled from the demiplane. The PCs can go with it, replacing the lord, and retire to the "happily ever after" of making the land rich and green, once more.
