The hellburners were constructed by the Italian engineer Federigo Giambelli, who had been hired and subsidised by Elizabeth I of England, unofficially supporting the rebels, to assist the city. In the winter of 1585, Antwerp was besieged by the army of Alexander Farnese, the commander of the Habsburg forces in the Spanish Netherlands, who had constructed a ship bridge over the River Scheldt near Kalloo between Antwerp and the sea, to starve the population by blockade; it had been completed on 25 February. To supply the city it was imperative to destroy the ship bridge.