Genghis_Sean
High School English teacher
The Samurai Invasion of Korea
I didn't purchase the Korean DLC until recently and bought it in an effort to check off the achievements it offered. I wasn't able to find any walkthroughs for it here, so here's my breakdown for how to complete it. I started with the hardest one first, and am now working my way down. I began with playing as the Koreans on deity and won it handily, though I quit the first couple times as the Japanese overran me no matter what I did. I only later realized they were supposed to. Here are several principles one should understand for how to win the scenario:
1) Understand that the Japanese will overrun the entirely of the southern half of the peninsula no matter what. You'll have a half dozen units maybe and they'll have 30, so make your decisions accordingly. They managed to get all the way to Seoul, my capital, and take it before I was able to make a stand. What this means is that you should make wise decisions about your production. If you can complete a happiness improvement, I'd do that since it won't benefit Japan any once they own the city, and eventually you'll be taking the city back and happiness will be an issue. You can consider quickly completing a unit, but I suspect none will complete quickly enough, so you might need to settle for gold per turn. You'll hold Seoul long enough to build something but few others.
2) Forget the battles and win the war. By that, I mean don't subscribe to the kamikaze way of thinking, but flee to fight another day. Pull out your units and retreat to good defensible positions and then flee again. Eventually you'll be getting reinforcements that spontaneously show up, and China will send help as well.
3) Trebuchets and Turtle boats will win you the game. Trebuchets in particular are invaluable because they can grab high ground and pick off the Japanese as they approach. Try to build a cluster of 3-4 that you protect and along with a great general will allow them to upgrade to the point that they can cripple or kill in one shot. All other ground units are expendable.
Your navy is equally important because you'll be so heavily outnumbered. To kill a ground unit in combat often takes 2-3 units. If you finish it off with a melee unit, you often lose it the following turn because it is exposed. With turtle boats, you can kill a melee unit crossing the straight with a single attack. Therefore, you should designate at least one city to constantly crank out naval units. You'll want a cluster of at least four ships on each side of the peninsula to deal with the Japanese ships and to sink as many embarked units as you can so you don't have to fight them on land later.
4) Finally, focus on keeping a key core of units alive throughout the scenario and gaining them as much experience as possible. Principly, these should be trebuchets and ships. Don't worry about buying city attack for the trebuchets. They'll be used much more often to kill troops. Keep your ships in action constantly. If you don't have a naval action at hand, seek out landed foes to shell and gain more experience. Eventually, your elite units will take control of the scenario.
I never felt time pressure to win the game. Retaking cities starts slowly and then finishes in a rush. I finished with 17 turns to spare even on deity, so it wasn't that hard as long as you keep your head about you and don't get discouraged by how badly you're overrun at the start of the scenario. I'm finishing the scenario as Japan on Immortal setting later today. I'll do a writeup of that before too long.

Edit: Achievements earned:
Fear the Turtle Win the Korean scenario as Korea
Honoring the Ancestors Win Korean scenario on diety
Taekwon-DOH Lose the Korean scenario as Korea (by retiring at anytime)
Seoul Power Win before 100 turns as any power
I didn't purchase the Korean DLC until recently and bought it in an effort to check off the achievements it offered. I wasn't able to find any walkthroughs for it here, so here's my breakdown for how to complete it. I started with the hardest one first, and am now working my way down. I began with playing as the Koreans on deity and won it handily, though I quit the first couple times as the Japanese overran me no matter what I did. I only later realized they were supposed to. Here are several principles one should understand for how to win the scenario:
1) Understand that the Japanese will overrun the entirely of the southern half of the peninsula no matter what. You'll have a half dozen units maybe and they'll have 30, so make your decisions accordingly. They managed to get all the way to Seoul, my capital, and take it before I was able to make a stand. What this means is that you should make wise decisions about your production. If you can complete a happiness improvement, I'd do that since it won't benefit Japan any once they own the city, and eventually you'll be taking the city back and happiness will be an issue. You can consider quickly completing a unit, but I suspect none will complete quickly enough, so you might need to settle for gold per turn. You'll hold Seoul long enough to build something but few others.
2) Forget the battles and win the war. By that, I mean don't subscribe to the kamikaze way of thinking, but flee to fight another day. Pull out your units and retreat to good defensible positions and then flee again. Eventually you'll be getting reinforcements that spontaneously show up, and China will send help as well.
3) Trebuchets and Turtle boats will win you the game. Trebuchets in particular are invaluable because they can grab high ground and pick off the Japanese as they approach. Try to build a cluster of 3-4 that you protect and along with a great general will allow them to upgrade to the point that they can cripple or kill in one shot. All other ground units are expendable.
Your navy is equally important because you'll be so heavily outnumbered. To kill a ground unit in combat often takes 2-3 units. If you finish it off with a melee unit, you often lose it the following turn because it is exposed. With turtle boats, you can kill a melee unit crossing the straight with a single attack. Therefore, you should designate at least one city to constantly crank out naval units. You'll want a cluster of at least four ships on each side of the peninsula to deal with the Japanese ships and to sink as many embarked units as you can so you don't have to fight them on land later.
4) Finally, focus on keeping a key core of units alive throughout the scenario and gaining them as much experience as possible. Principly, these should be trebuchets and ships. Don't worry about buying city attack for the trebuchets. They'll be used much more often to kill troops. Keep your ships in action constantly. If you don't have a naval action at hand, seek out landed foes to shell and gain more experience. Eventually, your elite units will take control of the scenario.
I never felt time pressure to win the game. Retaking cities starts slowly and then finishes in a rush. I finished with 17 turns to spare even on deity, so it wasn't that hard as long as you keep your head about you and don't get discouraged by how badly you're overrun at the start of the scenario. I'm finishing the scenario as Japan on Immortal setting later today. I'll do a writeup of that before too long.

Edit: Achievements earned:
Fear the Turtle Win the Korean scenario as Korea
Honoring the Ancestors Win Korean scenario on diety
Taekwon-DOH Lose the Korean scenario as Korea (by retiring at anytime)
Seoul Power Win before 100 turns as any power