The Koreans are mainly a defensive civilization with strong naval and gunpowder capablities.
Their ability to gather stone faster than other civilizations allows them to build up their defense quickly. Their towers upgrade automatically and gain +1 bonus range in the Castle Age and another one in the Imperial Age, allowing them to attack units from further away and take the enemy down before they get too close. Also, if against an aggressive team such as Celts or Goths, it is a good idea to try and establish defensive buildings around their base as early as possible to disrupt their economy. With each advancing age the range of Korean towers increases as well as upgrading automatically, make it increasingly detrimental to the enemy. Their unique technology increases the range of Mangonels.
The Koreans already have an innate bonus that improves Mangonel range so this adds even more to it, allowing them to outrange towers and castles. This lets them substitute Mangonels for Trebuchets and Bombard Cannons if the player is low on resources, as they are much more expensive. The War Wagon, the Korean unique unit, is a powerful and heavily armored but somewhat slow cavalry archer. They are effective support units and can last a long time in battle due to their durability.
The Koreans also have a strong navy and their second unique unit, the Turtle Ship, is very powerful and can take down and easily destroy enemy navies even if outnumbered. The Koreans have access to all gunpowder units, though they cannot research the Elite Cannon Galleon upgrade. They can also research most infantry and archer upgrades allowing them to wield an effective foot army.
The main weakness of the Koreans are their inability to research many Monastery technologies, most importantly Heresy and Redemption. Also they are somewhat weak at attacking. In contrast with their defensive bonuses they lack any offensive bonuses that can potentially turn the tide of a battle. As such they are a disadvantage when taking the fight to an enemy. Also their cavalry is somewhat weak, missing the Paladin and Camel in addition to the Bloodlines technology. As with the Vikings due to their naval unique unit the Koreans are missing one of the mainline ships, in this case the Demolition Ship. However the large attack and durability of the Turtle Ship makes up for this.