Mortars have been used for hundreds of years. The earliest mortars were used in Korea in a 1413 naval battle when Korean gunsmiths developed the "wan'gu" (gourd-shaped mortar) (완구, 碗口). The earliest version of the "wan'gu" dates back to 1407. Choi Hae-san (최해산, 崔海山) (1380–1443), the son of Choe Mu-seon(최무선) (1325–1395), is generally credited with inventing the "wan'gu". The first use in siege warfare was at the 1453 siege of Constantinople by Mehmed the Conqueror. An Italian account of the 1456 siege of Belgrade by Giovanni da Tagliacozzo states that the Ottoman Turks used seven mortars that fired "stone shots one Italian mile high". The time of flight of these was apparently long enough that casualties could be avoided by posting observers to give warning of their trajectories.