The turtle ship has been credited as being the first more or less iron clad ship in the world though this is not entirely sure. Amongst other things they were used in the wars between Korea and Japan.

Sources indicate that sharp iron spikes protruded from hexagonal plates covering the top of the turtle ship. While it is clear from the available sources that the top of the ship was covered with iron spikes to prevent boarding, there is insufficient evidence to support the claim that it was actually iron plated.

The turtle ship had ten oars and eleven gun ports on each side. Usually, there was one gun port in the dragon head's mouth. There were two more gun ports in the bow and stern of the turtle ship. The heavy cannons enabled the turtle ships to unleash a volley of cannonballs (some would use wooden bolts several feet in length, with specially engineered iron fins).

An advantage of the closed deck was that it protected the Korean sailors and marines from small arms and incendiary fire. The spikes discouraged Japanese sailors from engaging in their primary method of naval combat at the time, grappling an enemy ship with hooks and then boarding it to engage in hand-to-hand combat.

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Detail: pikes on the deck.

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Replica of a Geobukseon in the War Memorial Museum in Seoul.

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Turtle ships are a beloved subject in Korean history. Here they are depicted on a 500 Won note from 1966.

The Turtle ship was a heavy, invincible Korean battle ship type.

The turtle ship was technically a hull that was placed on top of a Panokseon (a type of rowing/sailing ship), with a large anchor kept in the bow of the ship, and other minor modifications.

Class: Panokseon
Length: 100-120 ft
Beam: 30-40 ft
Propulsion: 80 oarsmen
Extra crew: 50 soldiers
Armament: flame thrower using sulphur, iron spikes (deck)
11 cannon (range 200- 600 yards)

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Depiction of a Geobukseon from 1795, based on an image from 1592.