Mining
Digging under a wall, propping the tunnel with timber, then burning the props to bring the wall down.
Miners, or sappers, started a tunnel out of sight, often under cover of a shed, and dug beneath the foundations of a tower or wall. They held up the roof with wooden props. When the chamber was large enough they packed it with brushwood and fat and set it alight. As the props burned away, the tunnel collapsed and the masonry above it cracked or fell. After about 1500, gunpowder charges replaced fire.
Mining was slow and only worked where the ground could be dug: rock, water-filled moats and marshes defeated it. Defenders listened for digging, sometimes watching bowls of water for ripples, and dug counter-mines to break into the enemy tunnel and fight underground. Round towers and thick, sloping bases made mining harder.
The most famous English example is Rochester in 1215, where the king's miners brought down a corner of the great keep.