It was recognized that the gate of Tongres was not the most assailable, but rather the strongest portion of the defences, and Alexander therefore determined to shift his batteries to the gate of Bois-le-Duc.

At the same time, the attempt upon that of Tongres was to be varied, but not abandoned.

Four thousand miners, who had passed half their lives in burrowing for coal in that anthracite region, had been furnished by the Bishop of Liege, and this force was now set to their subterranean work.

A mine having been opened at a distance, the besiegers slowly worked their way towards the Tongres gate, while at the same time the more ostensible operations were in the opposite direction.

The besieged had their miners also, for the peasants in the city had been used to work with mattock and pickaxe.