Johannes Grant was an engineer employed by the Byzantine Empire

Byzantine Empire

The Byzantine Empire was the Eastern Roman Empire during the periods of Late Antiquity and the Middle Ages, centred on the capital of Constantinople. Known simply as the Roman Empire or Romania to its inhabitants and neighbours, the Empire was the direct continuation of the Ancient Roman State...

 at the fall of Constantinople

Fall of Constantinople

The Fall of Constantinople was the capture of the capital of the Byzantine Empire, which occurred after a siege by the Ottoman Empire, under the command of Ottoman Sultan Mehmed II, against the defending army commanded by Byzantine Emperor Constantine XI...

 in 1453. Contemporary Greek accounts referred to him as being German

Germans

The Germans are a Germanic ethnic group native to Central Europe. The English term Germans has referred to the German-speaking population of the Holy Roman Empire since the Late Middle Ages....

, although more recent scholarship has suggested he may actually have been Scottish

Scotland

Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

, named "John Grant". His use of counter-tunnelling prevented the Turks from weakening or invading Constantinople from under the walls.