18th century engraving of wheeled siege towers and bridges used by Frederick II of Germany, Holy Roman Emperor for the siege of Jerusalem. The Sixth Crusade started in 1228 as an attempt to regain Jerusalem. It began seven years after the failure of the Fifth Crusade. It involved very little actual fighting. The diplomatic maneuvering of the Holy Roman Emperor, Frederick II resulted in the Kingdom of Jerusalem regaining control of Jerusalem and other areas for fifteen years. Siege towers were used to get troops over an enemy curtain wall. When a siege tower was near a wall, it would drop a gangplank between it and the wall. Troops could then rush onto the walls and into the castle or city.