The so-called "turtle ship" (Korean kobukson) was a kind of warship famously employed by legendary Korean admiral Yi Sun-shin to defend Korea from Toyotomi Hideyoshi's invasions from 1592 to 1598. The turtle ship got its nickname from the fact that it looked like, well, a turtle. Officially the ship was named "large ramming ship".
The turtle ship was essentially a modified version of the standard Korean naval vessel of the day, the panokson, with a rounded, spiked, iron-plated turtleshell-like roof added for protection. Completing the turtle-like appearance of the craft was the "dragon head" which protruded from under the "shell" at the front of the craft. In some versions of the ship, the head emitted noxious smoke from its mouth to strike fear into the enemy, and in other versions it contained a small cannon in the mouth.
The ship was propelled primarily by oars with occasional help from two sails on two masts, and was heavily armed with several cannon peeking out from under the shell on all sides. The main weapon of the ship, however, was the massive iron and wooden ram at the front.
First invented in the early 1400s for use against Japanese Pirates, the turtle ships were devastatingly effective against the pirates and later Hideyoshi's navy. The Japanese did not make much use of cannon, so the main Japanese naval tactic of the time consisted of shooting some arrows and then maneuvering close enough to the enemy to board her and finish off her crew with swords. The turtle ship proved completely impervious to Japanese arrows as well as impossible to board, and was thus a truly unstoppable weapon.
The only hope the Japanese ships had against the turtle ship was to try to run away from it, as the turtle ship was relatively slow. However, with its stable, rounded bottom, the turtle ship was incredibly maneuverable, able to spin rapidly about on its own axis and change direction at will. Its numerous guns also allowed it to cripple Japanese ships and prevent them from running while it closed in for the final death ram.
Things were made a bit more fair by the fact that the turtle ship was costly and time-consuming to build. Admiral Yi never actually had more than two of these things at one time. However, he was extremely clever at maximizing their capabilities. A favorite tactic would be to show the Japanese only normal panokson ships while the turtle boats lay in wait around the bend. The Korean ships would briefly engage and then pretend to flee. The hotheaded glory-hound samurai would then pursue only to run smack into the turtle boats, and by then it would be too late.
Turtle ships continued to be used by the Royal Korean Navy all the way up until well into the 18th century, although there were never more than three or four at a time, at most. Today the turtle ship is a beloved Korean icon, and practically a national symbol of the Korean people and their patriotic defiance of foreign power. Appropriately, a visitor to Korea will find that the image of the turtle ship now appears on all manner of souvenirs and tchotchkes.