As soon as he had secured a base of operations in Hainaut and Artois, Farnese set himself in earnest to the task of reconquering Brabant and Flanders by force of arms, beginning with Maastricht. Farnese commenced the Siege of Maastricht on 12 March 1579. He ordered his troops to sap the walls. The inhabitants of Maastricht too were digging to reach the Spanish tunnels. Deep underground the fighting continued, hundreds of Spanish soldiers died as boiling oil was poured into their tunnels. Others died because of a lack of oxygen when the Dutch defenders ignited fires within them. Another 500 Spanish soldiers died when a mine, which they planned to use to blow up the wall, exploded prematurely.