The Goryeo Navy had a tactics dependent on large sized battleship. Large battleship was profitable in arrow shooting, which was the Goryeo Army's Traditional Skill, and the use of war instruments. On the other hand, Waegu(倭寇) preferred to boarding tactics with small and rapid ship.
Since 1350, Waegu began to invade in Goryeo on large scale. The Goryeo Navy could not match them. In generally we have an idea that Goryeo failed to repulse invader because they flighted in land not sea. It is a miss understanding. The Goryeo Government knew to have to depend Waegu in the sea. But they could not do because Waegu overwhelmed the Goryeo's Navy in scale, although the Goryeo's Tactics depending bow and large ship requested more soldiers and ships.
At last, King Gongmin(恭愍王) attempted to build battleships. But the Traditional Tactics needed too many ship beyond nation's capacity. To solve this problem, it is required to device a new tactics that was able to increase navy's power, especially in supremacy to ascending Waegu.
Two plans were attempted. The first was employing marines from the coast province or island. They were dispatched for education of fighting skills and usage of new weapons to soldiers
The second way, more important, was using of firearms. A cannon was little useful. It's power was weak and it had short range. More important weapons were the devices to attack enemies who intended to boarding. Hwatong(火桶) and Jilryeo-po(蒺藜砲) had good effects on attacking them, and compensated Goryeo navy's long timed weakness.
This reformation succeeded. The Goryeo's Naval Tactics depending large sized battleship was changed to very good system against the Waegu. The Joseon succeeded to this reformation course. It became the a primary factor for overcoming Japanese invasion in the 16th century.