Long post warning. I'm sorry.
"Education Fever"
Libraries and Universities cost +1 upkeep, but produce +25% science
Well, I think they cost upkeep anyway. This strikes me as an extremely modern feature of Korean culture as well (and isn't really something instantly recognizable, especially considering the historical importance of the French education system and Indian education system, which are ignored in the game).
May I share I few thoughts on your ideas?
Absolutely. This is usually why I post my ideas. I know they're extremely flawed and need feedback.
One thing that always bugged me about how the hwach'a were implemented is that, historically, hwach'a weren't meant to be used as siege weapons against cities. They were meant to be used against large formations of troops. I know it goes against the grain to give a UU a penalty, but I think hwach'a should be less effective against hard targets (cities). In the interest of gameplay, maybe have them the same strength as cannon but just give them a bonus against troops outside of cities? That would still make them pretty powerful.
The problem is picking the exact amount to scale it up. Cannons are 26, Artillery is 32 (I feel going from 26 to 30 isn't very dramatic, while going to 32 is overly dramatic). Then again, Artillery does have more range and, since this wouldn't be against cities, maybe it would balance out.
BTW, the Spike TV show Deadliest Warrior tested the Chinese equivalent called the nest of bees (I understand Myth Busters tested a Hwach'a, but I haven't been able to see the episode). The thing looked really, really impressive. But, interestingly, each arrow seemed to swerve around their enemy and no one was scathed (which would probably just be a coincidence). I do agree that it was an anti-infantry weapon.
I like the idea of turtle ships in general, but they predated widespread use of cannons on ships (at least in East Asia). The armor was mostly there to defend against enemy archers, I believe.
Wikipedia seems to disagree with you (yeah, wikipedia, but the article seems trustworthy). Apparently Japan didn't really have naval batteries, which could be what you're thinking of.
More importantly, though, I'd like to see Korea have a UU/UB combination, rather than two UUs. Especially during the Joseon period (if we're going with Sejong the Great--and I agree, by the way), Korea was not a very warlike nation, even though they did have to fight off a number of invasions. I would keep the hwach'a over the turtle ship, simply because they probably had a greater effect (turtle ships were indeed used, but it was the knowledge of local waters and the tactical genius of the naval commanders (Yi Sun-sin) that allowed the Korean navy to defeat the Japanese). For a UB, maybe something that represents Korean scientific achievements (like Cheomseongdae (observatory)), or something that would make cities on hills even stronger (to represent the mountain fortresses here). Those are just some rough ideas, though.
I wasn't specifically going for the Joseon period, things just seemed to coincidentally go that way. I chose two UUs for a couple reasons. One, I had suggested a lot of UU/UB pairs and was getting bored recommending them. Two, I felt that one of Civ5's distinctive features is the possibility of 2 UUs so I tried to pick them when possible. I felt Firaxis basically chose which civs to give 2 UUs based on how much knowledge they had of them. This led, generally, to western civs getting 2 UUs and Eastern Civs (China, India, Siam) getting a UU and a UB. Japan was unique (because it's better in the west). I was trying to remove Japan's uniqueness and reduce accidental Eurocentrism this way.
But I do want to address the Cheomseongdae (which is a mouthful, btw). I do like it. Since it's the oldest observatory in the world, I wouldn't have it replace observatories (more like have it replace monuments and give science as well). Another suggestion would be the Seowon, which they used in Civ4. I like your idea better because it gives Korea more diversity than the Joseon period. Although Seowon is easier to pronounce
Overall, I think there are both merits to 2 UUs and a UU/UB system. I feel it's almost easier to think of the latter, though, so I try to suggest as many civs for 2 UUs as I can think of (provided the units are unique). It's not often I have enough knowledge of Asian units to be able to do that
As far as the UA goes, hmm... either something that deals with culture or science, I would think. I'm not sure I have a complete enough understanding of the workings of UAs to propose anything balanced yet, though.
Heh, I guess I didn't really contribute anything specific there. Just thinking aloud.
That's OK, it's what I generally do. When I think of Korea, I have a habit of thinking "culture" as well, but that's so vague. Usually what I've been trying to do is think of some term, cultural characteristic, event, or idea that stands out as being truly belonging to that civ (if I can't, I fudge it, some are harder than others). Then I'll either try and think of an interesting game mechanic that broadly mirrors the idea (or at least you can convince someone who doesn't know about the term that it mirrors it) or, if it doesn't mirror the idea, generally reflects how you expect Korea to play.
For game mechanics (a name definitely helps narrow things down, but we'll leave that aside for now), we would have to think how Korea should play. While Japan and China seem very warlike, Korea seems geared towards a peaceful player. They have one UU that isn't effective at taking cities, but could decimate enemy units in the field (better for defense, it seems). They'll either have a peaceful scientific UB or a naval unit that is more suited for coastal defense than offense (can destroy enemy ships, but, once again, isn't better at bombarding cities). Now you've mentioned how they had to fight off a lot of invaders and you've mentioned how they had mountain forts. Both of these things could suggest a defensive Unique Ability. They also had ties to Confucianism and that bureaucratic system. I could see some kind of cultural thing as well (afterall, you can't just play peacefully, you need a possible victory condition).
So I think finding a fitting name for something Korean (either in a specific Korean thing or even an English name that would describe this characteristic in Korea) for either a defensive or scientific/cultural UA would be the next stage.