Built in the mid-15th century as the biggest double-towered gate on the waterfront, it also served to move heavy cargoes directly onto or off the vessels. For this purpose two large wheels – 5m in diameter – were installed as a hoist with a rope wound around the axle; it was put in motion by people ‘walking’ along the inner circumference of the wheels as a treadmill. It could hoist loads of up to 2000kg, making it the biggest crane in medieval Europe. At the beginning of the 17th century another set of wheels was added higher up, for installing masts.