Antwerp was one of the richest cities in northern Europe and a rebel stronghold ever since Spanish and Walloon troops sacked it in 1576. The city was open to the sea, strongly fortified, and well defended under the leadership of Marnix van St. Aldegonde. Engineer Sebastian Baroccio cut off all access to the sea by constructing a bridge of boats across the Scheldt. The Dutch sailed fireships, called Hellburners, against the bridge and one of the exploding infernal machines blew up a 200-foot-long span and killed 800 Spaniards. The besiegers repaired the damage, however, and pressed the investment. The city surrendered in 1585 as 60,000 Antwerp citizens (60% of the pre-siege population) fled north. Brussels, Mechelen and Geertruidenberg fell the same year.